feat(workers): bus-backed Workers panel (registry, control, installed flag)

Ships the backend half of Config → Workers:

* Worker registry aggregates `system.*.health` + `system.bus.health`
  heartbeats into a last-seen dict; OK / STALE / UNKNOWN tiers drop
  out of a 90s window (3× the 30s heartbeat interval).
* `GET /api/v1/workers` returns the snapshot plus `bus_connected`
  (so the UI can explain "all UNKNOWN" when the bus socket is down)
  and a per-row `installed` flag populated from
  `systemctl list-unit-files decnet-*.service` (cached 30s).
* `POST /api/v1/workers/{name}/stop` publishes a stop intent on
  `system.<name>.control`; workers listen via the shared control
  listener in `bus/publish.py`.
* Heartbeat + control listener wired into collector / profiler /
  sniffer / prober / mutator worker loops. API self-heartbeats too
  so the panel always has one ground-truth row.
* Topic helper `system_control(name)` + tests covering builder
  validation, control listener shutdown path, and the API surface
  (auth gating, bus-connected field, unknown-name 404).

Adds `StartFailure` / `StartAllResponse` models in anticipation of
the upcoming start endpoints (DEBT-034).

decnet/mutator/engine.py

@@ -27,6 +27,7 @@ from decnet.bus.base import BaseBus
 from decnet.bus.factory import get_bus
 from decnet.bus.publish import (
     publish_safely as _publish_safely,
+    run_control_listener_signal as _run_control_listener_signal,
     run_health_heartbeat as _run_health_heartbeat,
 )
 from decnet.mutator.events import MutationTrigger, emit_decky_mutated
@@ -332,6 +333,11 @@ async def run_watch_loop(repo: BaseRepository, poll_interval_secs: int = 10) ->
         heartbeat_task = asyncio.create_task(
             _run_health_heartbeat(bus, "mutator"),
         )
+        # Control listener: SIGTERM-based so the existing shutdown path
+        # (cancel wake_tasks + heartbeat_task) runs unchanged.
+        wake_tasks.append(asyncio.create_task(
+            _run_control_listener_signal(bus, "mutator"),
+        ))
     except Exception as exc:  # noqa: BLE001
         log.warning("mutator: bus unavailable, running in poll-only mode: %s", exc)