feat(cli): cross-backend list active + --backend flag + dashboard picker (issue #77)

CLI and dashboard now share one cross-backend abstraction for
listing + launching bottles, so adding a backend (docker /
smolmachines) lights up in both places without separate wiring.

Backend abstraction:
- New `ActiveBottle` dataclass (`backend_name`, `slug`,
  `agent_name`, `started_at`, `services`) replaces the
  docker-specific `ActiveAgent`. Same field surface for the
  existing dashboard consumers; `ActiveAgent` becomes a typed
  alias for source-compat.
- New `BottleBackend.enumerate_active() -> Sequence[ActiveBottle]`
  replaces the old `list_active() -> None` (which printed and
  returned nothing). Docker implements it via the existing
  compose query; smolmachines implements it via `smolvm machine
  ls --json` cross-referenced with each bundle container's
  `CLAUDE_BOTTLE_SIDECAR_DAEMONS` env (`backend/smolmachines/
  enumerate.py`).
- New `enumerate_active_bottles()` and `known_backend_names()`
  module-level helpers fold every backend into one call.
- `get_bottle_backend(name=None)` takes an optional explicit
  name (precedence: arg > $CLAUDE_BOTTLE_BACKEND > "docker").

CLI:
- `./cli.py list active` enumerates every backend, prints
  tab-separated `<backend>\t<slug>\t<agent>\t<services>`. The
  smolmachines bottle the user was looking for now shows up.
- `./cli.py start` grows `--backend=<docker|smolmachines>`
  (choices pulled live from `known_backend_names()`). Threaded
  through `prepare_with_preflight(backend_name=...)` so the
  resume path picks up the flag too.

Dashboard:
- Active agents pane lists both backends (the row formatter now
  prefixes `[docker]` / `[smolmachines]`).
- New-agent flow inserts a backend picker modal between agent
  pick and preflight (`_backend_picker_modal`). Short-circuits
  when only one backend is registered.
- `discover_active_agents()` collapses to
  `enumerate_active_bottles()`; `_parse_services_by_project` and
  `_query_services_by_project` move to
  `backend/docker/cleanup.py` where the docker enumerator owns
  them.

Tests: parser + enumerate-active tests relocated to
`test_docker_enumerate_active.py`. New
`test_backend_selection.py` covers `get_bottle_backend`,
`known_backend_names`, `enumerate_active_bottles`. New
`test_cli_start_backend_flag.py` covers `--backend`'s argparse
shape + the explicit-over-env precedence.

605 unit tests pass.

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

claude_bottle/backend/docker/cleanup.py

@@ -29,10 +29,16 @@ 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
-from .compose import COMPOSE_PROJECT_PREFIX, list_compose_projects
+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,
+)
 
 
 def _list_prefixed_containers() -> list[str]:
@@ -161,24 +167,67 @@ def cleanup(plan: DockerBottleCleanupPlan) -> None:
             warn(f"failed to remove {path}: {e}")
 
 
-def list_active() -> None:
-    """Print every active claude-bottle compose project + its
-    services. Empty banner when there are none."""
-    docker_mod.require_docker()
-    active = list_compose_projects(include_stopped=False)
-    if not active:
-        info("no active claude-bottle compose projects")
-        return
-    print()
-    for project in active:
-        info(f"compose project: {project}")
-        ps = subprocess.run(
-            ["docker", "compose", "-p", project, "ps", "--format",
-             "{{.Service}}\t{{.Name}}\t{{.Status}}"],
+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,
         )
-        for line in (ps.stdout or "").splitlines():
-            service, _, rest = line.partition("\t")
-            name, _, status = rest.partition("\t")
-            info(f"  {service:12s}  {name}  ({status})")
-    print()
+    except FileNotFoundError:
+        return {}
+    if r.returncode != 0:
+        return {}
+    return _parse_services_by_project(r.stdout or "")