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/cleanup.py

@@ -1,4 +1,4 @@
-"""Cleanup + active-listing for the Docker bottle backend.
+"""Cleanup for the Docker bottle backend.
 
 PRD 0018 chunk 4: cleanup is centered on `docker compose ls`.
 Pre-compose code paths could leave bare containers / networks
@@ -18,8 +18,8 @@ scan, just as a fallback bucket alongside the project list.
 
 `cleanup` removes everything in the plan.
 
-`list_active` queries the same compose project namespace and prints
-each project's services for ad-hoc inspection.
+Active-agent enumeration lives in `backend/docker/enumerate.py`
+(mirror of `backend/smolmachines/enumerate.py`).
 """
 
 from __future__ import annotations
@@ -29,16 +29,10 @@ import subprocess
 
 from ... import supervise as _supervise
 from ...log import info, warn
-from .. import ActiveBottle
 from . import util as docker_mod
 from .bottle_cleanup_plan import DockerBottleCleanupPlan
-from .bottle_state import bottle_state_dir, is_preserved, read_metadata
-from .compose import (
-    COMPOSE_PROJECT_PREFIX,
-    compose_project_name,
-    list_active_slugs,
-    list_compose_projects,
-)
+from .bottle_state import bottle_state_dir, is_preserved
+from .compose import COMPOSE_PROJECT_PREFIX, list_compose_projects
 
 
 def _list_prefixed_containers() -> list[str]:
@@ -167,67 +161,6 @@ def cleanup(plan: DockerBottleCleanupPlan) -> None:
             warn(f"failed to remove {path}: {e}")
 
 
-def enumerate_active() -> list[ActiveBottle]:
-    """All currently-running docker-backed bottles as
-    `ActiveBottle` records. Backend-agnostic shape — the CLI
-    `list active` command and the dashboard agents pane both
-    consume this. Empty list when docker is unreachable or
-    nothing's running."""
-    # docker on PATH? Defensive — `list active` shouldn't die
-    # just because the docker backend isn't usable on this host.
-    if shutil.which("docker") is None:
-        return []
-    slugs = list_active_slugs(include_stopped=False)
-    if not slugs:
-        return []
-    services_by_project = _query_services_by_project()
-    out: list[ActiveBottle] = []
-    for slug in slugs:
-        project = compose_project_name(slug)
-        services = services_by_project.get(project, set())
-        metadata = read_metadata(slug)
-        out.append(ActiveBottle(
-            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, ...}}`. Moved
-    here from the dashboard so the same query backs the CLI's
-    `list active` and the dashboard's agents pane."""
-    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 "")
+# `enumerate_active` moved to `backend/docker/enumerate.py` to
+# mirror the smolmachines layout. Cleanup keeps the orphan
+# enumeration; enumeration of live agents is its own concern.