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/cli/list.py

@@ -3,8 +3,9 @@
 from __future__ import annotations
 
 import argparse
+import sys
 
-from ..backend import get_bottle_backend
+from ..backend import enumerate_active_bottles
 from ..manifest import Manifest
 from ._common import PROG, USER_CWD
 
@@ -20,5 +21,17 @@ def cmd_list(argv: list[str]) -> int:
             print(name)
         return 0
 
-    get_bottle_backend().list_active()
+    # `active` enumerates every backend (docker + smolmachines)
+    # so smolmachines bottles aren't hidden behind the env var.
+    active = enumerate_active_bottles()
+    if not active:
+        print("no active claude-bottle bottles", file=sys.stderr)
+        return 0
+    # One line per bottle: `<backend>\t<slug>\t<agent>\t<status>`.
+    # Tab-separated keeps the format stable for shell pipelines;
+    # the dashboard renders the same data through its own
+    # formatter.
+    for b in active:
+        services = ",".join(b.services) if b.services else "-"
+        print(f"{b.backend_name}\t{b.slug}\t{b.agent_name}\t{services}")
     return 0