style(backend): drop stale "moved/removed" pointer comments

PR #78 review comments 580, 582, 584. Each was a comment
describing what the previous refactor removed or relocated —
information that's in git history, not load-bearing for a
reader of the file as-is.

- claude_bottle/backend/docker/cleanup.py: drop trailing
  "enumerate_active moved to enumerate.py" note.
- tests/unit/test_dashboard_active_agents.py: drop module
  docstring paragraph about which tests moved where.
- tests/unit/test_docker_enumerate_active.py: drop
  "noop-when-docker-missing lives at the cross-backend gate
  now" trailing comment.

607 tests still pass.

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

tests/unit/test_docker_enumerate_active.py

@@ -173,13 +173,6 @@ class TestEnumerateActive(_FakeHomeMixin, unittest.TestCase):
             [a.slug for a in active],
         )
 
-    # `noop when docker missing` lives at the cross-backend gate
-    # now (`enumerate_active_agents` skips backends whose
-    # `is_available()` reports False — see
-    # `test_backend_selection.TestEnumerateActiveAgents`). This
-    # module assumes docker is available when called, matching the
-    # smolmachines/enumerate.py contract.
-
 
 if __name__ == "__main__":
     unittest.main()