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/selection.py

@@ -0,0 +1,188 @@
+"""Backend registry, selection, and active-agent enumeration.
+
+Resolves which bottle backend to use (explicit name / `BOT_BOTTLE_BACKEND` /
+auto-select), and enumerates running agents across every available backend. The
+three concrete backends are imported lazily inside `_get_backends` so this
+module — and anything that only needs to *select* a backend — stays cheap.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from typing import Any
+
+from ..log import die, info, warn
+from ..util import read_tty_line
+from .base import ActiveAgent, BottleBackend
+
+
+# _backends is None until the first call to _get_backends(), at which
+# point all three concrete backend classes are imported and instantiated.
+# Keeping the imports out of module scope means that importing any
+# backend sub-module (e.g. `backend.docker.util`) no longer drags the
+# firecracker and macos-container implementations into memory.
+#
+# Tests may replace _backends with a {name: fake} dict via patch.object;
+# _get_backends() returns the current module-level value as-is when it
+# is not None, so test fakes take effect without triggering real imports.
+_backends: dict[str, BottleBackend[Any, Any]] | None = None
+
+
+def _get_backends() -> dict[str, BottleBackend[Any, Any]]:
+    """Return the registry of all backend instances, loading lazily on first call."""
+    global _backends  # pylint: disable=global-statement
+    if _backends is None:
+        from .docker import DockerBottleBackend
+        from .firecracker import FirecrackerBottleBackend
+        from .macos_container import MacosContainerBottleBackend
+        _backends = {
+            "docker": DockerBottleBackend(),
+            "firecracker": FirecrackerBottleBackend(),
+            "macos-container": MacosContainerBottleBackend(),
+        }
+    return _backends
+
+
+def get_bottle_backend(
+    name: str | None = None,
+    *,
+    prompt: bool = True,
+) -> BottleBackend[Any, Any]:
+    """Resolve the bottle backend.
+
+    `name` precedence:
+      1. explicit arg (e.g. resume passes the recorded backend name)
+      2. BOT_BOTTLE_BACKEND env var
+      3. auto-selection: VM backend first, docker fallback with prompt
+
+    `prompt` controls whether auto-selection may block on an interactive
+    [i/d/q] prompt when falling back to docker. Pass `prompt=False` in
+    non-interactive contexts (headless launches, CI) so the call dies
+    with an actionable message instead of hanging.
+
+    Dies with a pointer at the known backends if the chosen name
+    isn't implemented."""
+    resolved = name or os.environ.get("BOT_BOTTLE_BACKEND")
+    if resolved is None:
+        resolved = _auto_select_backend(prompt=prompt)
+    backends = _get_backends()
+    if resolved not in backends:
+        known = ", ".join(sorted(backends))
+        die(f"unknown backend {resolved!r}; known backends: {known}")
+    return backends[resolved]
+
+
+def _platform_vm_suggestion() -> str:
+    """Platform-appropriate VM backend name for install suggestions."""
+    return "macos-container" if sys.platform == "darwin" else "firecracker"
+
+
+def _print_vm_install_instructions() -> None:
+    """Print platform-appropriate VM backend install instructions to stderr."""
+    vm = _platform_vm_suggestion()
+    if vm == "macos-container":
+        info("Install Apple Container: https://github.com/apple/container/releases")
+        info("Then start the service:  container system start")
+    else:
+        info("Install Firecracker: https://github.com/firecracker-microvm/firecracker/releases")
+        info("Configure the host:  ./cli.py backend setup")
+
+
+def _auto_select_backend(prompt: bool = True) -> str:
+    """Tier-1 / tier-2 backend auto-selection.
+
+    Tier 1: VM backend — macos-container on macOS when Apple Container is
+    installed; firecracker on KVM-capable Linux even before the binary is
+    present (its preflight prints an install pointer).
+
+    Tier 2: docker, with a security warning and an interactive prompt.
+    When `prompt=False` (headless / CI), dies with an actionable message
+    instead of blocking on a TTY read. When docker is also absent, prints
+    VM install instructions and exits.
+    """
+    # --- Tier 1: VM backend -----------------------------------------
+    if has_backend("macos-container"):
+        return "macos-container"
+    # A KVM-capable Linux host defaults to firecracker even when the
+    # `firecracker` binary isn't installed yet: selecting it here routes
+    # start through firecracker's preflight, which prints an install
+    # pointer, instead of silently falling back to docker.
+    from .firecracker import FirecrackerBottleBackend
+    if FirecrackerBottleBackend.is_host_capable():
+        return "firecracker"
+
+    # --- Tier 2: docker fallback ------------------------------------
+    if not has_backend("docker"):
+        info("No backend available on this host.")
+        _print_vm_install_instructions()
+        die("no backend available; install a VM backend and re-run")
+
+    vm = _platform_vm_suggestion()
+    warn(
+        "docker is less secure than VM backends — "
+        "containers share the host kernel."
+    )
+    if not prompt:
+        die(
+            f"no VM backend available; set BOT_BOTTLE_BACKEND=docker to proceed "
+            f"with docker, or install the {vm!r} backend."
+        )
+    sys.stderr.write(
+        f"bot-bottle: For better isolation, install the {vm!r} backend.\n"
+        f"  [i] show {vm} install instructions and exit\n"
+        "  [d] use docker anyway\n"
+        "  [q] quit\n"
+        "bot-bottle: choice [i/d/q]: "
+    )
+    sys.stderr.flush()
+    reply = read_tty_line().strip().lower()
+    if reply == "d":
+        return "docker"
+    if reply == "i":
+        _print_vm_install_instructions()
+    die("not proceeding with docker; install a VM backend or set BOT_BOTTLE_BACKEND=docker")
+
+
+def known_backend_names() -> tuple[str, ...]:
+    """Sorted tuple of all backend keys in `_get_backends()`. Used by
+    argparse (`--backend` choices) and the dashboard's backend
+    picker."""
+    return tuple(sorted(_get_backends()))
+
+
+def has_backend(name: str) -> bool:
+    """Whether the named backend's runtime prerequisites are
+    available on the current host. Cross-backend callers (list,
+    cleanup) skip unavailable backends so a docker-only host
+    doesn't fail when the firecracker backend isn't usable,
+    and vice versa.
+
+    Returns False for unknown names so callers can pass
+    arbitrary input without separate validation."""
+    backends = _get_backends()
+    if name not in backends:
+        return False
+    return backends[name].is_available()
+
+
+def enumerate_active_agents() -> list[ActiveAgent]:
+    """All currently-running agents, across every available
+    backend. Used by CLI `list active` and the dashboard's agents
+    pane so neither has to know which backends exist. Skips
+    backends whose `is_available()` reports False.
+
+    Sorted by `(started_at, slug)` so the list is stable across
+    dashboard refresh ticks — agents don't shift position while
+    the operator navigates with arrow keys. ISO 8601 timestamps
+    sort lexicographically in chronological order; `slug` is the
+    deterministic tiebreaker. Agents with missing metadata
+    (`started_at == ""`) sort first."""
+    out: list[ActiveAgent] = []
+    backends = _get_backends()
+    for name in sorted(backends):
+        if not backends[name].is_available():
+            continue
+        out.extend(backends[name].enumerate_active())
+    out.sort(key=lambda a: (a.started_at, a.slug))
+    return out