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/enumerate.py

@@ -0,0 +1,80 @@
+"""Active-agent enumeration for the docker backend.
+
+Mirrors `backend/smolmachines/enumerate.py`: returns
+`ActiveAgent` records the CLI `list active` command and the
+dashboard agents pane consume. Empty when docker isn't reachable
+— gated by `has_backend('docker')` at the cross-backend caller
+so this module trusts that docker is available when called.
+
+The parser (`_parse_services_by_project`) is exposed for direct
+unit testing; the docker `docker ps` invocation is in
+`_query_services_by_project`."""
+
+from __future__ import annotations
+
+import subprocess
+
+from .. import ActiveAgent
+from .bottle_state import read_metadata
+from .compose import compose_project_name, list_active_slugs
+
+
+def enumerate_active() -> list[ActiveAgent]:
+    """All currently-running docker-backed agents. Caller is
+    responsible for gating on `has_backend('docker')` if it
+    matters; if docker is missing the `docker ps` call below
+    returns an empty list silently."""
+    slugs = list_active_slugs(include_stopped=False)
+    if not slugs:
+        return []
+    services_by_project = _query_services_by_project()
+    out: list[ActiveAgent] = []
+    for slug in slugs:
+        project = compose_project_name(slug)
+        services = services_by_project.get(project, set())
+        metadata = read_metadata(slug)
+        out.append(ActiveAgent(
+            backend_name="docker",
+            slug=slug,
+            agent_name=metadata.agent_name if metadata else "?",
+            started_at=metadata.started_at if metadata else "",
+            services=tuple(sorted(services)),
+        ))
+    return out
+
+
+def _parse_services_by_project(stdout: str) -> dict[str, set[str]]:
+    """Parse `docker ps` output formatted as
+    `<project-label>\\t<service-label>` (one line per container)
+    into a `{project: {service, ...}}` mapping. Pure function for
+    testing — the docker invocation is in `_query_services_by_project`."""
+    out: dict[str, set[str]] = {}
+    for line in stdout.splitlines():
+        project, _, service = line.partition("\t")
+        if not project or not service:
+            continue
+        out.setdefault(project, set()).add(service)
+    return out
+
+
+def _query_services_by_project() -> dict[str, set[str]]:
+    """One `docker ps` call → `{project: {service, ...}}`. Used
+    by the CLI's `list active` and the dashboard's agents pane —
+    one subprocess per refresh tick, not one per bottle."""
+    try:
+        r = subprocess.run(
+            [
+                "docker", "ps",
+                "--filter", "label=com.docker.compose.project",
+                "--format",
+                '{{.Label "com.docker.compose.project"}}'
+                "\t"
+                '{{.Label "com.docker.compose.service"}}',
+            ],
+            capture_output=True, text=True, check=False,
+        )
+    except FileNotFoundError:
+        return {}
+    if r.returncode != 0:
+        return {}
+    return _parse_services_by_project(r.stdout or "")