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/smolmachines/enumerate.py

@@ -1,26 +1,30 @@
-"""Active-bottle enumeration for the smolmachines backend (PRD
+"""Active-agent enumeration for the smolmachines backend (PRD
 0023 chunk 4 follow-up + issue #77).
 
-Returns a list of `ActiveBottle` records — same shape the docker
+Returns a list of `ActiveAgent` records — same shape the docker
 backend produces — so CLI `list active` and the dashboard agents
 pane render both backends through one code path.
 
-A smolmachines bottle is "active" when its smolvm guest is
+A smolmachines agent is "active" when its smolvm guest is
 running. We cross-reference against the per-bottle sidecar
 bundle container to populate the `services` field (which daemons
 are up in the bundle); without a bundle we still surface the VM
-so the operator can see + clean it up."""
+so the operator can see + clean it up.
+
+The cross-backend caller gates on `has_backend("smolmachines")`
+and `has_backend("docker")`, so this module assumes both are
+available when called. Both subprocess calls below still
+tolerate "command not on PATH" defensively, but the gate is the
+intended access pattern."""
 
 from __future__ import annotations
 
 import json
-import shutil
 import subprocess
 
-from .. import ActiveBottle
+from .. import ActiveAgent
 from ..docker.bottle_state import read_metadata
 from . import sidecar_bundle as _bundle
-from . import smolvm as _smolvm
 
 
 # Smolvm VM names produced by prepare are `claude-bottle-<slug>`,
@@ -29,12 +33,12 @@ from . import smolvm as _smolvm
 _VM_NAME_PREFIX = "claude-bottle-"
 
 
-def enumerate_active() -> list[ActiveBottle]:
-    """All currently-running smolmachines-backed bottles. Empty
-    list when smolvm isn't on PATH or no matching VMs are
-    running."""
-    if not _smolvm.is_available():
-        return []
+def enumerate_active() -> list[ActiveAgent]:
+    """All currently-running smolmachines-backed agents. Empty
+    list when no matching VMs are running. Caller is responsible
+    for gating on `has_backend('smolmachines')` if needed; if
+    smolvm is missing the `smolvm machine ls` call below returns
+    nothing silently."""
     result = subprocess.run(
         ["smolvm", "machine", "ls", "--json"],
         capture_output=True, text=True, check=False,
@@ -46,7 +50,7 @@ def enumerate_active() -> list[ActiveBottle]:
     except json.JSONDecodeError:
         return []
     services_by_slug = _query_bundle_services()
-    out: list[ActiveBottle] = []
+    out: list[ActiveAgent] = []
     for m in machines:
         name = m.get("name") or ""
         state = m.get("state") or ""
@@ -54,7 +58,7 @@ def enumerate_active() -> list[ActiveBottle]:
             continue
         slug = name[len(_VM_NAME_PREFIX):]
         metadata = read_metadata(slug)
-        out.append(ActiveBottle(
+        out.append(ActiveAgent(
             backend_name="smolmachines",
             slug=slug,
             agent_name=metadata.agent_name if metadata else "?",
@@ -69,8 +73,17 @@ def _query_bundle_services() -> dict[str, tuple[str, ...]]:
     bundle container's `CLAUDE_BOTTLE_SIDECAR_DAEMONS` env var.
     Smolmachines bundles all run the PRD-0024 image with the
     same daemon set declared via env, so one inspect per bundle
-    gets us the picture without exec'ing into the container."""
-    if shutil.which("docker") is None:
+    gets us the picture without exec'ing into the container.
+
+    Returns an empty mapping when the docker backend isn't
+    available — the bundle services field on each ActiveAgent
+    just shows up empty, matching the docker backend's "starting"
+    state."""
+    # Late import: `has_backend` lives on the backend package's
+    # __init__, which imports this module transitively. Pulling
+    # the name in at call time sidesteps the cycle.
+    from .. import has_backend
+    if not has_backend("docker"):
         return {}
     ps = subprocess.run(
         ["docker", "ps",