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/backend.py

@@ -7,10 +7,11 @@ 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 enumerate as _enumerate
 from . import launch as _launch
 from . import prepare as _prepare
+from . import smolvm as _smolvm
 from .bottle import SmolmachinesBottle
 from .bottle_cleanup_plan import SmolmachinesBottleCleanupPlan
 from .bottle_plan import SmolmachinesBottlePlan
@@ -29,6 +30,14 @@ class SmolmachinesBottleBackend(
 
     name = "smolmachines"
 
+    @classmethod
+    def is_available(cls) -> bool:
+        """`smolvm` on PATH. The backend additionally needs macOS
+        for libkrun + TSI, but `enumerate_active` / `cleanup` are
+        host-shell ops that gracefully no-op on Linux too — the
+        runtime check happens at `prepare`."""
+        return _smolvm.is_available()
+
     def _resolve_plan(
         self, spec: BottleSpec, *, stage_dir: Path
     ) -> SmolmachinesBottlePlan:
@@ -74,5 +83,5 @@ class SmolmachinesBottleBackend(
         # Nothing to clean in chunks 1-3 — see
         # SmolmachinesBottleCleanupPlan docstring.
 
-    def enumerate_active(self) -> Sequence[ActiveBottle]:
+    def enumerate_active(self) -> Sequence[ActiveAgent]:
         return _enumerate.enumerate_active()