refactor(backend): split the heavy backend/__init__ into base + selection, thin the package init

backend/__init__.py eagerly imported the whole framework (manifest, egress,
git-gate, env, workspace, agent-provider), so importing ANY backend.* submodule
paid ~32 modules just to run the package init. Split it:

  - backend/base.py     — the abstract contract (BottleSpec, BottlePlan,
                          BottleCleanupPlan, ExecResult, ActiveAgent, Bottle,
                          BottleImages, BottleBackend). Carries the framework
                          imports; loaded only when a caller needs the contract.
  - backend/selection.py — backend registry / selection / enumeration
                          (get_bottle_backend, _get_backends, _auto_select_backend,
                          has_backend, known_backend_names, enumerate_active_agents).
                          Concrete backends still imported lazily inside.
  - backend/__init__.py  — thin: a __getattr__ that resolves the public names
                          from those submodules on first access (+ a TYPE_CHECKING
                          block for checkers). Existing `from bot_bottle.backend
                          import X` and patch.object call-sites keep working.

`import bot_bottle.backend` drops from 32 -> 2 bot_bottle modules. Repointed the
backend-selection test's patch targets to the `selection` module (the functions
reference each other there now; the FirecrackerBottleBackend class-method
patches are unchanged — same class object).

Note: backend.docker.util is still heavy because backend/docker/__init__ eagerly
imports DockerBottleBackend — slimming the three sub-package inits is the
follow-up that makes the leaves cheap.

Full unit suite green (2243).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/base.py

@@ -0,0 +1,607 @@
+"""The abstract backend contract (PRD 0018 / 0070).
+
+The backend-neutral types every bottle backend implements: the launch
+`BottleSpec`, the `BottlePlan` / `BottleCleanupPlan` ABCs, the running-`Bottle`
++ `ExecResult` shapes, `BottleImages`, and the `BottleBackend` ABC itself.
+
+This carries the framework imports (manifest, egress, git-gate, env, workspace,
+agent-provider) the contract's signatures and helpers need — which is why it
+lives here rather than in `backend/__init__.py`: touching an unrelated
+`backend.*` module then doesn't drag the whole framework into memory. The
+thin package `__init__` re-exports these names lazily.
+"""
+
+from __future__ import annotations
+
+import os
+import shlex
+import sys
+from abc import ABC, abstractmethod
+from contextlib import AbstractContextManager, contextmanager
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Generator, Generic, Sequence, TypeVar
+
+from ..agent_provider import AgentProvisionPlan, get_provider, build_agent_provision_plan
+from ..egress import EgressPlan
+from ..git_gate import GitGatePlan
+from ..log import die, info
+from ..util import expand_tilde
+from ..manifest import Manifest, ManifestIndex
+from ..supervisor.plan import SupervisePlan
+from ..env import resolve_env, ResolvedEnv
+from ..workspace import WorkspacePlan, workspace_plan
+from .print_util import print_multi, visible_agent_env_names
+from .util import host_skill_dir
+
+
+
+@dataclass(frozen=True)
+class BottleSpec:
+    """CLI-supplied intent. Backend-agnostic — each backend's prepare
+    step consumes it and produces its own backend-specific plan.
+    Resolved values (image names, container name, scratch paths, runsc
+    availability) live on the plan, not the spec."""
+
+    manifest: ManifestIndex
+    agent_name: str
+    copy_cwd: bool
+    user_cwd: str
+    # PRD 0016 follow-up: when set, the backend's prepare step uses
+    # this identity instead of minting a fresh one — the resume path
+    # (`cli.py resume <identity>`) sets this to continue an existing
+    # bottle's state. Empty string for a fresh `start`.
+    identity: str = ""
+    label: str = ""
+    color: str = ""
+    # Ordered bottle names selected at launch (issue #269). When non-empty
+    # they are merged in order and replace the agent's `bottle:` field.
+    bottle_names: tuple[str, ...] = ()
+    # True when launched via --headless (no TTY, no interactive prompts).
+    # The git-gate host-key preflight uses this to error rather than prompt.
+    headless: bool = False
+    # Image startup policy. "fresh" preserves the normal build path;
+    # "cached" reuses the current local image/artifact without rebuilding.
+    image_policy: str = "fresh"
+
+
+@dataclass(frozen=True)
+class BottlePlan(ABC):
+    """Base output of a backend's prepare step. Concrete subclasses
+    (e.g. DockerBottlePlan) add backend-specific resolved fields."""
+
+    spec: BottleSpec
+    manifest: Manifest
+    stage_dir: Path
+    git_gate_plan: GitGatePlan
+
+    @property
+    def guest_home(self) -> str:
+        return self.agent_provision.guest_home
+
+    @property
+    def git_gate_insteadof_host(self) -> str:
+        """Host (and optional port) used in git-gate insteadOf URLs.
+        Docker uses the compose-network DNS alias; VM backends may
+        override with an IP:port when the guest has no DNS."""
+        return "git-gate"
+
+    @property
+    def git_gate_insteadof_scheme(self) -> str:
+        """URL scheme for git-gate insteadOf rewrites. 'git' for
+        Docker (git daemon); VM backends may override (e.g. 'http'
+        over a published host port)."""
+        return "git"
+    egress_plan: EgressPlan
+    supervise_plan: SupervisePlan | None
+    agent_provision: AgentProvisionPlan
+
+    @property
+    def workspace_plan(self) -> WorkspacePlan:
+        return workspace_plan(self.spec, guest_home=self.guest_home)
+
+    def print(self) -> None:
+        """Render the y/N preflight summary to stderr."""
+        spec = self.spec
+        manifest = self.manifest
+        agent = manifest.agent
+        bottle = manifest.bottle
+
+        env_names = visible_agent_env_names(
+            sorted(
+                set(bottle.env.keys())
+                | set(self.agent_provision.guest_env.keys())
+            ),
+            hidden_env_names=self.agent_provision.hidden_env_names,
+        )
+
+        print(file=sys.stderr)
+        info(f"agent           : {spec.agent_name}")
+        info(f"provider        : {self.agent_provision.template}")
+        print_multi("env             ", env_names)
+        print_multi("skills          ", list(agent.skills))
+        effective_bottles = (
+            list(spec.bottle_names) if spec.bottle_names
+            else ([agent.bottle] if agent.bottle else [])
+        )
+        print_multi("bottle          ", effective_bottles)
+
+        identity = manifest.git_identity_summary()
+        if identity:
+            info(f"  git identity  : {identity}")
+
+        git_lines = [
+            f"{u.name} → {u.upstream_host}:{u.upstream_port}"
+            for u in self.git_gate_plan.upstreams
+        ]
+        if git_lines:
+            print_multi("  git gate      ", git_lines)
+
+        if self.egress_plan.routes:
+            egress_lines = []
+            for r in self.egress_plan.routes:
+                auth = f" [auth:{r.auth_scheme}]" if r.auth_scheme else ""
+                egress_lines.append(f"{r.host}{auth}")
+            print_multi("  egress        ", egress_lines)
+        print(file=sys.stderr)
+
+
+@dataclass(frozen=True)
+class BottleCleanupPlan(ABC):
+    """Base output of a backend's prepare_cleanup step. Concrete
+    subclasses (e.g. DockerBottleCleanupPlan) carry backend-specific
+    lists of resources to be removed and implement `print` + `empty`."""
+
+    @abstractmethod
+    def print(self) -> None:
+        """Render the cleanup y/N summary to stderr."""
+
+    @property
+    @abstractmethod
+    def empty(self) -> bool:
+        """True iff there is nothing to clean up; the CLI uses this to
+        short-circuit before showing the y/N."""
+
+
+@dataclass(frozen=True)
+class ExecResult:
+    """Captured result of `Bottle.exec`. Backend-neutral: the Docker
+    impl populates it from a `subprocess.CompletedProcess`, but a
+    VM backend could populate it from any source that produces a
+    returncode + captured streams."""
+
+    returncode: int
+    stdout: str
+    stderr: str
+
+
+@dataclass(frozen=True)
+class ActiveAgent:
+    """One currently-running agent, as the CLI `list active` and
+    dashboard agents pane render it. ("Agent" is the project's
+    consistent name for the thing running inside a bottle — the
+    bottle is the container, the agent is what runs in it.)
+
+    Fields are deliberately backend-neutral. `services` is the set
+    of gateway daemons currently up for this bottle (`egress`,
+    `git-gate`, `supervise`); the dashboard uses it to
+    gate edit verbs. `backend_name` is the matching key in
+    `_BACKENDS` (`docker` / `firecracker` / `macos-container`) — used by the active-
+    list rendering to disambiguate and by the dashboard's
+    re-attach path."""
+
+    backend_name: str
+    slug: str
+    agent_name: str       # from metadata.json; "?" if missing
+    started_at: str       # ISO 8601 from metadata.json; "" if missing
+    services: tuple[str, ...]  # alphabetical
+    label: str = ""
+    color: str = ""
+
+
+class Bottle(ABC):
+    """Handle to a running bottle. Yielded by a backend's launch step.
+
+    `exec_agent` runs the selected agent CLI inside the bottle and
+    blocks until the session ends. `exec` runs a POSIX shell script inside the bottle
+    and returns the captured result. `cp_in` copies a host path into
+    the bottle. `close` is an idempotent alias for context-manager
+    teardown.
+    """
+
+    name: str
+
+    @abstractmethod
+    def agent_argv(
+        self, argv: list[str], *, tty: bool = True,
+    ) -> list[str]:
+        """Return the host-side argv that runs the selected agent
+        inside the bottle. Used by `exec_agent` for foreground
+        handoffs and by the dashboard's tmux `respawn-pane` flow,
+        which needs the argv up front (it spawns claude in a tmux
+        pane rather than as a child of the current process).
+
+        Implementations transparently inject
+        `--append-system-prompt-file` when the bottle was launched
+        with a provisioned prompt path."""
+        ...
+
+    @abstractmethod
+    def exec_agent(self, argv: list[str], *, tty: bool = True) -> int: ...
+
+    @abstractmethod
+    def exec(self, script: str, *, user: str = "node") -> ExecResult:
+        """Run `script` as a POSIX shell script inside the bottle as
+        `user` (default `node`, matching the agent image's USER
+        directive) and return the captured stdout/stderr/returncode.
+        The bottle's environment (including HTTPS_PROXY pointing at
+        the egress daemon) is inherited by the child. Non-zero
+        exit does not raise — callers inspect `returncode`
+        themselves.
+
+        Pass `user="root"` for shell-outs that need privileged file
+        writes / package install — provisioning calls that need root
+        bypass `Bottle.exec` and use the backend-specific raw
+        machine-exec helper, but the tests have a legitimate use
+        case for arbitrary-user runs."""
+
+    @abstractmethod
+    def cp_in(self, host_path: str, container_path: str) -> None: ...
+
+    @abstractmethod
+    def close(self) -> None: ...
+
+
+
+
+PlanT = TypeVar("PlanT", bound=BottlePlan)
+CleanupT = TypeVar("CleanupT", bound=BottleCleanupPlan)
+
+
+@dataclass(frozen=True)
+class BottleImages:
+    """Resolved image references (or artifact paths) for a bottle launch.
+
+    For Docker/macOS-container backends, `agent` and `sidecar` are string
+    image refs. For the smolmachines backend they are Path objects pointing
+    to pre-built `.smolmachine` artifacts."""
+
+    agent: str | Path
+    sidecar: str | Path = ""
+
+
+class BottleBackend(ABC, Generic[PlanT, CleanupT]):
+    """Abstract base for selectable bottle backends. Concrete subclasses
+    (e.g. DockerBottleBackend) own their own prepare/launch impls.
+    Parameterized over the backend's concrete plan + cleanup-plan types
+    so subclass methods get the narrow type without isinstance
+    boilerplate."""
+
+    name: str
+
+    # Whether this backend can run a container engine *inside* the bottle.
+    # Backends that cannot must reject `nested_containers: true` rather than
+    # reach for a host daemon socket (issue #392).
+    supports_nested_containers: bool = False
+
+    def prepare(self, spec: BottleSpec, stage_dir: Path) -> PlanT:
+        """Template method: run cross-backend host-side validation, then
+        delegate to the subclass's `_resolve_plan` for the
+        backend-specific resolution (names, scratch files, etc.). The
+        validation step is enforced here so a future backend cannot
+        accidentally skip it. No remote/runtime resources are created."""
+        from .resolve_common import (
+            merge_provision_env_vars,
+            mint_slug,
+            prepare_agent_state_dir,
+            prepare_egress,
+            prepare_git_gate,
+            prepare_supervise,
+            reject_nested_containers,
+            resolve_manifest_dockerfile,
+            write_launch_metadata,
+        )
+
+        manifest = self._validate(spec)
+
+        if not self.supports_nested_containers:
+            reject_nested_containers(self.name, manifest)
+
+        self._preflight()
+
+        from ..git_gate_host_key import preflight_host_keys
+        manifest = preflight_host_keys(
+            manifest,
+            headless=spec.headless,
+            home_md=spec.manifest.home_md,
+        )
+
+        manifest_bottle = manifest.bottle
+        manifest_agent_provider = manifest_bottle.agent_provider
+        agent_provider = get_provider(manifest_agent_provider.template)
+        resolved_env = resolve_env(manifest)
+        workspace = workspace_plan(spec, guest_home=agent_provider.guest_home)
+
+        slug = mint_slug(spec)
+        write_launch_metadata(slug, spec, compose_project="", backend=self.name)
+
+        # Manifest may override the Dockerfile per-bottle; otherwise fall
+        # back to the provider plugin's bundled Dockerfile (next to its
+        # agent_provider.py module).
+        if manifest_agent_provider.dockerfile:
+            agent_dockerfile_path = resolve_manifest_dockerfile(
+                manifest_agent_provider.dockerfile, spec,
+            )
+        else:
+            agent_dockerfile_path = str(agent_provider.dockerfile)
+
+        agent_dir, prompt_file = prepare_agent_state_dir(slug, manifest)
+
+        agent_provision_plan = build_agent_provision_plan(
+            template=manifest_agent_provider.template,
+            dockerfile=agent_dockerfile_path,
+            state_dir=agent_dir,
+            instance_name=f"bot-bottle-{slug}",
+            prompt_file=prompt_file,
+            guest_env=self._build_guest_env(resolved_env),
+            forward_host_credentials=manifest_agent_provider.forward_host_credentials,
+            auth_token=manifest_agent_provider.auth_token,
+            host_env=dict(os.environ),
+            trusted_project_path=workspace.workdir,
+            label=spec.label,
+            color=spec.color,
+            provider_settings=manifest_agent_provider.settings,
+        )
+        agent_provision_plan = merge_provision_env_vars(agent_provision_plan)
+        egress_plan = prepare_egress(manifest_bottle, slug, agent_provision_plan)
+        supervise_plan = prepare_supervise(manifest_bottle, slug)
+        git_gate_plan = prepare_git_gate(manifest_bottle, slug)
+
+        return self._resolve_plan(
+            spec,
+            manifest=manifest,
+            slug=slug,
+            resolved_env=resolved_env,
+            agent_provision_plan=agent_provision_plan,
+            egress_plan=egress_plan,
+            supervise_plan=supervise_plan,
+            git_gate_plan=git_gate_plan,
+            stage_dir=stage_dir,
+        )
+
+    def _build_guest_env(self, resolved_env: ResolvedEnv) -> dict[str, str]:
+        return {}
+
+    def _preflight(self) -> None:
+        """
+        tasks to do before resolving a plan
+        """
+        pass
+
+    def _validate(self, spec: BottleSpec) -> Manifest:
+        """Cross-backend pre-launch checks. Parses the selected agent and
+        its bottle (raising ManifestError on invalid content), confirms
+        skills are present on the host, and every git IdentityFile resolves.
+
+        Returns the loaded Manifest for the selected agent. Subclasses with
+        additional preconditions should override and call
+        `super()._validate(spec)` first."""
+        manifest = spec.manifest.load_for_agent(spec.agent_name, spec.bottle_names)
+        self._validate_skills(manifest.agent.skills)
+        self._validate_agent_provider_dockerfile(spec, manifest)
+        return manifest
+
+    def _validate_skills(self, skills: Sequence[str]) -> None:
+        """Each named skill must be a directory under the host's
+        `~/.claude/skills/`. The check is purely host-side, so the
+        default impl covers every backend."""
+        for name in skills:
+            path = host_skill_dir(name)
+            if not os.path.isdir(path):
+                die(
+                    f"skill '{name}' not found on host at {path}. "
+                    f"Create it under ~/.claude/skills/, then re-run."
+                )
+
+    def _validate_agent_provider_dockerfile(self, spec: BottleSpec, manifest: Manifest) -> None:
+        bottle = manifest.bottle
+        dockerfile = bottle.agent_provider.dockerfile
+        if not dockerfile:
+            return
+        path = Path(expand_tilde(dockerfile))
+        if not path.is_absolute():
+            path = Path(spec.user_cwd) / path
+        if not path.is_file():
+            effective = (
+                ", ".join(spec.bottle_names) if spec.bottle_names else manifest.agent.bottle
+            )
+            die(
+                f"agent_provider.dockerfile for bottle "
+                f"'{effective}' not found: {path}"
+            )
+
+    @abstractmethod
+    def _resolve_plan(self,
+                      spec: BottleSpec,
+                      *,
+                      manifest: Manifest,
+                      slug: str,
+                      resolved_env: ResolvedEnv,
+                      agent_provision_plan: AgentProvisionPlan,
+                      egress_plan: EgressPlan,
+                      git_gate_plan: GitGatePlan,
+                      supervise_plan: SupervisePlan | None,
+                      stage_dir: Path) -> PlanT:
+        """Backend-specific plan resolution: image/container names,
+        env-file, prompt-file, proxy plan, runtime detection. Called by
+        `prepare` after `_validate` succeeds. Instance name, image,
+        prompt file, Dockerfile path, and guest home all live on
+        `agent_provision_plan` — the source of truth."""
+
+    def prelaunch_checks(self, plan: PlanT) -> None:
+        """Raise StaleImageError if any cached image used by this plan is stale.
+        No-op default; backends override to call the shared check_stale*
+        helpers on their image/artifact timestamps. Called by the CLI before
+        launch so the operator can be prompted outside the launch context."""
+
+    @contextmanager
+    def launch(self, plan: PlanT) -> Generator[Bottle, None, None]:
+        """Template: build or load images, then delegate to _launch_impl."""
+        images = self._build_or_load_images(plan)
+        with self._launch_impl(plan, images) as bottle:
+            yield bottle
+
+    @abstractmethod
+    def _build_or_load_images(self, plan: PlanT) -> BottleImages:
+        """Return the agent and sidecar image references (or artifact paths)
+        for this plan, building fresh images when the policy requires it."""
+
+    @abstractmethod
+    def _launch_impl(self, plan: PlanT, images: BottleImages) -> AbstractContextManager[Bottle]:
+        """Bring up the bottle using pre-resolved images; yield a handle; tear down on exit."""
+
+    def provision(self, plan: PlanT, bottle: "Bottle") -> str | None:
+        """Copy host-side files (CA cert, prompt, skills, .git) into
+        the running bottle. Called from `launch` after the container
+        / machine is up. Returns the in-container prompt path if a
+        prompt was provisioned, else None — the Bottle handle uses it
+        to decide whether to add provider-specific prompt args to the
+        agent's argv.
+
+        Default orchestration: ca → prompt → provider apply → skills
+        → workspace → git → supervise-mcp. CA install runs first so
+        the agent's trust store is rebuilt before anything inside the
+        agent makes a TLS call.
+
+        Per PRD 0050 the per-provider steps (prompt, skills,
+        declarative provision-plan apply, supervise MCP registration)
+        live on the `AgentProvider` plugin. The backend only owns the
+        steps that are about backend infrastructure (CA, workspace,
+        git) and surfaces the supervise daemon URL its launch step
+        knows about via `supervise_mcp_url`.
+
+        PRD 0017: cred-proxy's agent-side dotfile rewrites (~/.npmrc,
+        ~/.gitconfig insteadOf, tea config) are gone. Egress-proxy is
+        on the agent's HTTP_PROXY path so every tool that respects
+        HTTPS_PROXY (claude-code, git over HTTPS, npm, curl) is
+        intercepted without per-tool reconfiguration."""
+        provider = get_provider(plan.agent_provision.template)
+        provider.provision_ca(bottle, plan)
+        prompt_path = provider.provision_prompt(plan, bottle)
+        provider.provision(plan, bottle)
+        provider.provision_skills(plan, bottle)
+        self.provision_workspace(plan, bottle)
+        provider.provision_git(bottle, plan)
+        provider.provision_supervise_mcp(
+            plan, bottle, self.supervise_mcp_url(plan),
+        )
+        return prompt_path
+
+    def provision_workspace(self, plan: PlanT, bottle: "Bottle") -> None:
+        """Copy the operator workspace into the running bottle.
+
+        This is the only supported workspace-provisioning path: Docker
+        does not build a derived image containing the current
+        workspace."""
+        workspace = plan.workspace_plan
+        if not (workspace.enabled and workspace.copy_contents):
+            return
+
+        guest_parent = workspace.guest_path.rsplit("/", 1)[0] or "/"
+        guest_path = shlex.quote(workspace.guest_path)
+        guest_parent = shlex.quote(guest_parent)
+        owner = shlex.quote(workspace.owner)
+        mode = shlex.quote(workspace.mode)
+        info(f"copying {workspace.host_path} -> {bottle.name}:{workspace.guest_path}")
+        bottle.exec(
+            f"rm -rf {guest_path} && mkdir -p {guest_parent}",
+            user="root",
+        )
+        bottle.cp_in(str(workspace.host_path), workspace.guest_path)
+        bottle.exec(
+            f"chown -R {owner} {guest_path} && chmod {mode} {guest_path}",
+            user="root",
+        )
+
+    def supervise_mcp_url(self, plan: PlanT) -> str:
+        """Return the agent-side URL of the per-bottle supervise
+        gateway, or "" when this bottle has no gateway. The provider
+        plugin's `provision_supervise_mcp` uses it to register the
+        MCP entry inside the guest.
+
+        Default returns "" so backends without supervise support
+        don't have to implement it. Docker and firecracker override."""
+        del plan
+        return ""
+
+    def ensure_orchestrator(self) -> str:
+        """Bring up this backend's per-host orchestrator + shared gateway
+        (idempotent) and return the host-reachable control-plane URL.
+
+        This is the backend-agnostic bring-up entry point: `launch` calls
+        it as part of starting a bottle, and operator tools (`supervise`)
+        call it to start the control plane on demand when none is running
+        yet. Docker starts the orchestrator + gateway containers;
+        firecracker boots the infra VM. Backends with no orchestrator
+        (macos-container) die with a pointer — the default here."""
+        die(f"backend {self.name!r} has no orchestrator control plane")
+
+    @abstractmethod
+    def prepare_cleanup(self) -> CleanupT:
+        """Enumerate orphaned resources from previous bottles. No side
+        effects; safe to call before the y/N."""
+
+    @abstractmethod
+    def cleanup(self, plan: CleanupT) -> None:
+        """Remove everything described by the cleanup plan."""
+
+    @abstractmethod
+    def enumerate_active(self) -> Sequence[ActiveAgent]:
+        """Return every currently-running agent on this backend.
+        Empty when none. Backend-specific: docker queries `docker
+        compose ls`; firecracker cross-references its running gateway
+        containers against per-bottle metadata."""
+
+    @classmethod
+    @abstractmethod
+    def is_available(cls) -> bool:
+        """Whether this backend's runtime prerequisites are satisfied
+        on the current host. Docker → `docker` on PATH; firecracker →
+        Linux + KVM. Used by the cross-backend
+        `enumerate_active_agents` / `cmd_cleanup` to skip backends
+        the operator hasn't installed, so a docker-only host
+        doesn't fail when `cli.py list active` walks past
+        firecracker."""
+
+    @classmethod
+    @abstractmethod
+    def setup(cls) -> int:
+        """Emit this backend's one-time host setup — privileged network
+        pool, daemon bring-up, install pointers, etc. — as
+        host-appropriate config or commands. Prints to stdout/stderr and
+        returns a shell exit code (0 = nothing to report / success). A
+        backend that needs no host setup prints a short note and returns
+        0. Invoked generically by `./cli.py backend setup [--backend=…]`
+        so operators can provision any backend without a
+        backend-specific command. Classmethod (like `is_available`) —
+        it's a host query, not per-bottle state."""
+
+    @classmethod
+    @abstractmethod
+    def status(cls) -> int:
+        """Report whether this backend's prerequisites are satisfied on
+        the host — binaries, daemon reachability, network pool, range
+        conflicts, etc. Prints a human-readable summary; returns 0 when
+        the backend is ready to launch and non-zero when something is
+        missing. Invoked by `./cli.py backend status [--backend=…]`."""
+
+    @classmethod
+    @abstractmethod
+    def teardown(cls) -> int:
+        """Undo `setup()` — the inverse operation, surfaced as
+        `./cli.py backend teardown [--backend=…]` (uninstall). Symmetric
+        with setup: where setup is advisory (prints the privileged
+        commands / declarative config to apply), teardown prints the
+        commands / config change to remove the host prerequisites. A
+        backend with no host setup prints a short note and returns 0.
+        Not called by the launch path or the test suite."""