refactor(backend): has_backend() helper + docker/enumerate split + ActiveAgent rename

Addresses PR #78 review feedback:

- New `has_backend(name)` on the backend package + abstract
  `BottleBackend.is_available()` on each concrete subclass.
  Replaces inline `shutil.which("docker") is None` checks in
  docker/cleanup.py:178 and smolmachines/enumerate.py:73.
  Docker → `shutil.which("docker") is not None`; smolmachines
  → `smolvm.is_available()`. Cross-backend `enumerate_active_
  agents()` skips backends whose `is_available()` is False so a
  docker-only host doesn't fail when iterating past
  smolmachines (and vice versa).
- Move docker `enumerate_active` + parser helpers out of
  cleanup.py into a new `backend/docker/enumerate.py`, mirroring
  the smolmachines/enumerate.py layout. cleanup.py is now
  purely about prepare_cleanup / cleanup; the active-listing
  concern owns its own file.
- Drop the `ActiveAgent = ActiveBottle` alias in dashboard.py.
  The canonical name is `ActiveAgent` (the thing running inside
  a bottle is always called "agent" in this codebase; the bottle
  is the container). Renamed `enumerate_active_bottles` →
  `enumerate_active_agents` to match.

Tests:
- `test_backend_selection.TestEnumerateActiveAgents
  .test_skips_unavailable_backends` locks down the
  `is_available()`-gated iteration.
- New `TestHasBackend` covers `has_backend("docker")` consulting
  the backend's `is_available`, and unknown-name → False.
- Existing tests follow the rename; the docker-availability-
  side-effect test in `test_docker_enumerate_active` moves up
  to the cross-backend layer (where the gate lives now).

607 unit tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/backend.py

@@ -1,10 +1,11 @@
 """DockerBottleBackend — the Docker implementation of BottleBackend.
 
-This module is a thin façade. The real work lives in three siblings:
+This module is a thin façade. The real work lives in four siblings:
 
-  - prepare.py  — host-side resolution into a DockerBottlePlan
-  - launch.py   — bring-up + teardown context manager
-  - cleanup.py  — orphan enumeration, removal, and active listing
+  - prepare.py    — host-side resolution into a DockerBottlePlan
+  - launch.py     — bring-up + teardown context manager
+  - cleanup.py    — orphan enumeration + removal
+  - enumerate.py  — active-agent listing
 
 The base class's `prepare` template runs cross-backend host-side
 validation before calling `_resolve_plan` here.
@@ -12,12 +13,14 @@ validation before calling `_resolve_plan` here.
 
 from __future__ import annotations
 
+import shutil
 from contextlib import contextmanager
 from pathlib import Path
 from typing import Generator, Sequence
 
-from .. import ActiveBottle, BottleBackend, BottleSpec
+from .. import ActiveAgent, BottleBackend, BottleSpec
 from . import cleanup as _cleanup
+from . import enumerate as _enumerate
 from . import launch as _launch
 from . import prepare as _prepare
 from .bottle import DockerBottle
@@ -36,6 +39,15 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
 
     name = "docker"
 
+    @classmethod
+    def is_available(cls) -> bool:
+        """`docker` on PATH is sufficient; we don't probe `docker info`
+        eagerly because the cross-backend enumerator runs this on
+        every `list active` and we'd pay a subprocess per call. A
+        broken daemon will surface its own error during prepare /
+        launch."""
+        return shutil.which("docker") is not None
+
     def _resolve_plan(self, spec: BottleSpec, *, stage_dir: Path) -> DockerBottlePlan:
         return _prepare.resolve_plan(spec, stage_dir=stage_dir)
 
@@ -65,5 +77,5 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
     def cleanup(self, plan: DockerBottleCleanupPlan) -> None:
         _cleanup.cleanup(plan)
 
-    def enumerate_active(self) -> Sequence[ActiveBottle]:
-        return _cleanup.enumerate_active()
+    def enumerate_active(self) -> Sequence[ActiveAgent]:
+        return _enumerate.enumerate_active()