refactor: rename platform abstraction to backend

Across the package:
  - claude_bottle/platform/         -> claude_bottle/backend/
  - platform/docker/platform.py     -> backend/docker/backend.py
  - class BottlePlatform            -> BottleBackend
  - class DockerBottlePlatform      -> DockerBottleBackend
  - get_bottle_platform()           -> get_bottle_backend()
  - env var CLAUDE_BOTTLE_PLATFORM  -> CLAUDE_BOTTLE_BACKEND
  - dict _PLATFORMS                 -> _BACKENDS

"Backend" is shorter and more established as the term for a
pluggable strategy-pattern implementation. "Platform" was vague
(could mean OS, hardware, cloud) and mildly redundant — Docker is
itself a platform.

The previous PRD section claiming "the Backend protocol was
rejected" referred to a low-level run/exec/cp/network_connect
protocol; the name was never the reason. The PRD is updated to
describe that rejected design by shape rather than by name.

The bottle/agent concepts and the manifest schema are unchanged.

claude_bottle/backend/docker/backend.py

@@ -0,0 +1,402 @@
+"""DockerBottleBackend — the Docker implementation of BottleBackend.
+
+Methods:
+  .prepare(spec, stage_dir=...) -> DockerBottlePlan
+  .launch(plan) -> ContextManager[DockerBottle]
+  .prepare_cleanup() -> DockerBottleCleanupPlan
+  .cleanup(plan) -> None
+  .list_active() -> None
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+from contextlib import contextmanager
+from pathlib import Path
+from typing import Iterator
+
+from ... import pipelock
+from ... import skills as skills_mod
+from ... import ssh as ssh_mod
+from ...env_resolve import env_resolve
+from ...log import die, info
+from .. import BottleBackend, BottleCleanupPlan, BottlePlan, BottleSpec
+from . import network as network_mod
+from . import util as docker_mod
+from .bottle import DockerBottle
+from .bottle_cleanup_plan import DockerBottleCleanupPlan
+from .bottle_plan import DockerBottlePlan
+
+
+# Where the repo root lives, for `docker build` context. Computed once.
+_REPO_DIR = str(Path(__file__).resolve().parent.parent.parent.parent)
+
+
+class DockerBottleBackend(BottleBackend):
+    """Docker backend implementation. Selected by CLAUDE_BOTTLE_BACKEND
+    (default)."""
+
+    name = "docker"
+
+    def prepare(self, spec: BottleSpec, *, stage_dir: Path) -> DockerBottlePlan:
+        """Resolve names, validate, write scratch files. No Docker
+        resources are created; the only side effects are host-side
+        files under stage_dir and a probe of `docker info`."""
+        docker_mod.require_docker()
+
+        manifest = spec.manifest
+        manifest.require_agent(spec.agent_name)
+        agent = manifest.agents[spec.agent_name]
+        bottle = manifest.bottle_for(spec.agent_name)
+        bottle_name = agent.bottle
+
+        slug = docker_mod.slugify(spec.agent_name)
+
+        image = os.environ.get("CLAUDE_BOTTLE_IMAGE", "claude-bottle:latest")
+        derived_image = ""
+        runtime_image = image
+        if spec.copy_cwd:
+            derived_image = os.environ.get(
+                "CLAUDE_BOTTLE_DERIVED_IMAGE", f"claude-bottle:cwd-{slug}"
+            )
+            runtime_image = derived_image
+
+        default_container = f"claude-bottle-{slug}"
+        pinned_container = os.environ.get("CLAUDE_BOTTLE_CONTAINER", "")
+        container_name = pinned_container or default_container
+        container_name_pinned = bool(pinned_container)
+        suffix = 2
+        if container_name_pinned:
+            if docker_mod.container_exists(container_name):
+                die(
+                    f"container '{container_name}' already exists "
+                    f"(pinned via CLAUDE_BOTTLE_CONTAINER). "
+                    f"Remove it with 'docker rm -f {container_name}' or unset the override."
+                )
+        else:
+            while docker_mod.container_exists(container_name):
+                container_name = f"{default_container}-{suffix}"
+                suffix += 1
+                if suffix > 100:
+                    die(
+                        f"could not find a free container name after "
+                        f"{default_container}-99; clean up old containers with "
+                        f"'docker rm -f <name>'"
+                    )
+
+        if agent.skills:
+            skills_mod.skills_validate_all(list(agent.skills))
+        if bottle.ssh:
+            ssh_mod.ssh_validate_entries(bottle.ssh)
+
+        env_file = stage_dir / "agent.env"
+        args_file = stage_dir / "docker-args"
+        prompt_file = stage_dir / "prompt.txt"
+        pipelock_yaml_filename = "pipelock.yaml"
+        pipelock_yaml = stage_dir / pipelock_yaml_filename
+        env_file.write_text("")
+        env_file.chmod(0o600)
+        args_file.write_text("")
+        prompt_file.write_text("")
+        prompt_file.chmod(0o600)
+
+        pipelock.pipelock_write_yaml(manifest, bottle_name, pipelock_yaml)
+        env_resolve(manifest, spec.agent_name, env_file, args_file)
+        prompt_file.write_text(agent.prompt)
+
+        allowlist_summary = pipelock.pipelock_allowlist_summary(manifest, bottle_name)
+        use_runsc = docker_mod.runsc_available()
+
+        return DockerBottlePlan(
+            spec=spec,
+            stage_dir=stage_dir,
+            slug=slug,
+            container_name=container_name,
+            container_name_pinned=container_name_pinned,
+            image=image,
+            derived_image=derived_image,
+            runtime_image=runtime_image,
+            env_file=env_file,
+            args_file=args_file,
+            prompt_file=prompt_file,
+            pipelock_yaml_path=pipelock_yaml,
+            pipelock_yaml_filename=pipelock_yaml_filename,
+            allowlist_summary=allowlist_summary,
+            use_runsc=use_runsc,
+        )
+
+    @contextmanager
+    def launch(self, plan: BottlePlan) -> Iterator[DockerBottle]:
+        """Build, launch, and provision a Docker bottle. Teardown on exit."""
+        assert isinstance(plan, DockerBottlePlan), (
+            f"DockerBottleBackend.launch expects DockerBottlePlan, "
+            f"got {type(plan).__name__}"
+        )
+
+        state: dict[str, str] = {
+            "container": "",
+            "pipelock": "",
+            "internal_network": "",
+            "egress_network": "",
+        }
+
+        def teardown() -> None:
+            try:
+                if state["container"] and docker_mod.container_exists(state["container"]):
+                    subprocess.run(
+                        ["docker", "rm", "-f", state["container"]],
+                        stdout=subprocess.DEVNULL,
+                        stderr=subprocess.DEVNULL,
+                    )
+                    state["container"] = ""
+                if state["pipelock"]:
+                    pipelock.pipelock_stop(plan.slug)
+                    state["pipelock"] = ""
+                if state["internal_network"]:
+                    network_mod.network_remove(state["internal_network"])
+                    state["internal_network"] = ""
+                if state["egress_network"]:
+                    network_mod.network_remove(state["egress_network"])
+                    state["egress_network"] = ""
+            except BaseException:
+                # Teardown must not raise; swallow so the caller's
+                # __exit__ path can still propagate the original error.
+                pass
+
+        try:
+            docker_mod.build_image(plan.image, _REPO_DIR)
+            if plan.derived_image:
+                docker_mod.build_image_with_cwd(
+                    plan.derived_image, plan.image, plan.spec.user_cwd
+                )
+
+            state["internal_network"] = network_mod.network_create_internal(plan.slug)
+            state["egress_network"] = network_mod.network_create_egress(plan.slug)
+            state["pipelock"] = pipelock.pipelock_start(
+                plan.slug,
+                state["internal_network"],
+                state["egress_network"],
+                plan.stage_dir,
+                plan.pipelock_yaml_filename,
+            )
+
+            container = self._run_agent_container(plan, state["internal_network"])
+            state["container"] = container
+
+            prompt_path = self._provision_container(plan, container)
+
+            bottle = DockerBottle(container, teardown, prompt_path)
+            yield bottle
+        finally:
+            teardown()
+
+    def _run_agent_container(self, plan: DockerBottlePlan, internal_network: str) -> str:
+        """Build the `docker run` argv and execute it, handling
+        name-conflict races by incrementing the suffix (unless the name
+        was user-pinned). Returns the resolved container name."""
+        proxy_url = pipelock.pipelock_proxy_url(plan.slug)
+        docker_args: list[str] = [
+            "--rm", "-d",
+            "--name", plan.container_name,
+            "--network", internal_network,
+            "-e", f"HTTPS_PROXY={proxy_url}",
+            "-e", f"HTTP_PROXY={proxy_url}",
+            "-e", "NO_PROXY=localhost,127.0.0.1",
+        ]
+        if plan.use_runsc:
+            docker_args.extend(["--runtime", "runsc"])
+        if plan.env_file.stat().st_size > 0:
+            docker_args.extend(["--env-file", str(plan.env_file)])
+
+        # ARGS_FILE pairs (-e, NAME) line-by-line.
+        args_lines = plan.args_file.read_text().splitlines()
+        i = 0
+        while i < len(args_lines):
+            flag = args_lines[i]
+            i += 1
+            if not flag:
+                continue
+            if i >= len(args_lines):
+                break
+            vname = args_lines[i]
+            i += 1
+            docker_args.extend([flag, vname])
+
+        if plan.spec.forward_oauth_token:
+            os.environ["CLAUDE_CODE_OAUTH_TOKEN"] = os.environ["CLAUDE_BOTTLE_OAUTH_TOKEN"]
+            docker_args.extend(["-e", "CLAUDE_CODE_OAUTH_TOKEN"])
+
+        docker_args.extend([plan.runtime_image, "sleep", "infinity"])
+
+        info(f"starting container {plan.container_name} from {plan.runtime_image}")
+
+        container = plan.container_name
+        base_name = plan.container_name
+        suffix = 2
+        while True:
+            run_result = subprocess.run(
+                ["docker", "run", *docker_args],
+                capture_output=True,
+                text=True,
+            )
+            if run_result.returncode == 0:
+                return container
+            err_text = run_result.stderr
+            if plan.container_name_pinned or "is already in use" not in err_text:
+                sys.stderr.write(err_text + "\n")
+                die(f"docker run failed for container '{container}'")
+            if suffix > 100:
+                die(
+                    f"could not find a free container name after "
+                    f"{base_name}-99 retries; clean up old containers"
+                )
+            container = f"{base_name}-{suffix}"
+            suffix += 1
+            name_idx = docker_args.index("--name") + 1
+            docker_args[name_idx] = container
+            info(f"name conflict; retrying as {container}")
+
+    def _provision_container(self, plan: DockerBottlePlan, container: str) -> str | None:
+        """Copy prompt, skills, ssh keys, and (optionally) .git into the
+        running container. Returns the in-container prompt path if a
+        prompt was provisioned, else None — the Bottle handle uses it
+        to decide whether to add --append-system-prompt-file to
+        claude's argv."""
+        container_home = os.environ.get("CLAUDE_BOTTLE_CONTAINER_HOME", "/home/node")
+        in_container_prompt_path = f"{container_home}/.claude-bottle-prompt.txt"
+
+        subprocess.run(
+            ["docker", "cp", str(plan.prompt_file), f"{container}:{in_container_prompt_path}"],
+            stdout=subprocess.DEVNULL,
+            check=True,
+        )
+        # `docker cp` preserves host UID; re-own/mode as root so node
+        # can read its own mode-600 prompt regardless of host UID.
+        subprocess.run(
+            ["docker", "exec", "-u", "0", container, "chown", "node:node", in_container_prompt_path],
+            stdout=subprocess.DEVNULL,
+            check=True,
+        )
+        subprocess.run(
+            ["docker", "exec", "-u", "0", container, "chmod", "600", in_container_prompt_path],
+            stdout=subprocess.DEVNULL,
+            check=True,
+        )
+
+        agent = plan.spec.manifest.agents[plan.spec.agent_name]
+        if agent.skills:
+            skills_mod.skills_copy_into(container, list(agent.skills))
+
+        bottle = plan.spec.manifest.bottle_for(plan.spec.agent_name)
+        if bottle.ssh:
+            proxy_host_port = pipelock.pipelock_proxy_host_port(plan.slug)
+            ssh_mod.ssh_setup(container, plan.stage_dir, proxy_host_port, bottle.ssh)
+
+        if plan.spec.copy_cwd and Path(plan.spec.user_cwd, ".git").is_dir():
+            info(f"copying {plan.spec.user_cwd}/.git -> {container}:/home/node/workspace/.git")
+            subprocess.run(
+                ["docker", "cp", f"{plan.spec.user_cwd}/.git", f"{container}:/home/node/workspace/.git"],
+                stdout=subprocess.DEVNULL,
+                check=True,
+            )
+            subprocess.run(
+                [
+                    "docker", "exec", "-u", "0", container,
+                    "chown", "-R", "node:node", "/home/node/workspace/.git",
+                ],
+                stdout=subprocess.DEVNULL,
+                check=True,
+            )
+
+        return in_container_prompt_path if agent.prompt else None
+
+    # --- Cleanup ---
+
+    def prepare_cleanup(self) -> DockerBottleCleanupPlan:
+        """Enumerate all claude-bottle-prefixed containers (running or
+        stopped) and networks. No removals — caller confirms first."""
+        docker_mod.require_docker()
+
+        # `docker ps -a --filter name=...` uses regex matching; anchor at
+        # the start so we don't pick up containers that merely contain
+        # "claude-bottle-" mid-name.
+        cr = subprocess.run(
+            [
+                "docker", "ps", "-a",
+                "--filter", "name=^claude-bottle-",
+                "--format", "{{.Names}}",
+            ],
+            capture_output=True,
+            text=True,
+        )
+        containers = tuple(sorted(
+            line for line in (cr.stdout or "").splitlines() if line
+        ))
+
+        # `docker network ls --filter name=...` uses substring matching.
+        # "claude-bottle-" is specific enough that false positives are
+        # not a concern.
+        nr = subprocess.run(
+            [
+                "docker", "network", "ls",
+                "--filter", "name=claude-bottle-",
+                "--format", "{{.Name}}",
+            ],
+            capture_output=True,
+            text=True,
+        )
+        networks = tuple(sorted(
+            line for line in (nr.stdout or "").splitlines() if line
+        ))
+
+        return DockerBottleCleanupPlan(containers=containers, networks=networks)
+
+    def cleanup(self, plan: BottleCleanupPlan) -> None:
+        """Remove the containers and networks listed in the plan.
+        Containers first; networks would refuse to delete while
+        containers are still attached."""
+        assert isinstance(plan, DockerBottleCleanupPlan), (
+            f"DockerBottleBackend.cleanup expects DockerBottleCleanupPlan, "
+            f"got {type(plan).__name__}"
+        )
+        for name in plan.containers:
+            info(f"removing container {name}")
+            subprocess.run(
+                ["docker", "rm", "-f", name],
+                stdout=subprocess.DEVNULL,
+                stderr=subprocess.DEVNULL,
+            )
+        for name in plan.networks:
+            info(f"removing network {name}")
+            subprocess.run(
+                ["docker", "network", "rm", name],
+                stdout=subprocess.DEVNULL,
+                stderr=subprocess.DEVNULL,
+            )
+
+    # --- List ---
+
+    def list_active(self) -> None:
+        """Print all running claude-bottle containers (name + status).
+        Prints a single-line banner if there are none."""
+        docker_mod.require_docker()
+        result = subprocess.run(
+            [
+                "docker", "ps",
+                "--filter", "name=^claude-bottle-",
+                "--format", "{{.Names}}\t{{.Status}}",
+            ],
+            capture_output=True,
+            text=True,
+        )
+        containers = (result.stdout or "").strip()
+        if not containers:
+            info("no active claude-bottle containers")
+            return
+        print()
+        for line in containers.splitlines():
+            name, _, status = line.partition("\t")
+            info(f"container: {name}  status: {status}")
+        print()