feat(lifecycle): runner + strategies + bus topic

Add decnet.lifecycle package: pure orchestration layer that the
master API will invoke via asyncio.create_task to drive DeckyLifecycle
rows through pending -> running -> succeeded | failed without
holding an HTTP request open.

Strategy classes per (operation, transport):
- LocalDeployStrategy: master-resident, runs engine.deployer.deploy
  in a thread.
- SwarmDeployStrategy: shards by host_uuid, dispatches via
  AgentClient.deploy; worker drives terminal via heartbeat.
- LocalMutateStrategy: write_compose + compose up.
- SwarmMutateStrategy: AgentClient.mutate; worker drives terminal.

decnet.bus.topics gains decky_lifecycle(name) -> decky.<name>.lifecycle
plus DECKY_LIFECYCLE constant. Payload documented in the wiki
(separate commit). publish_safely keeps bus best-effort.

Nothing is wired to call this yet -- next commits convert worker
/deploy /mutate to 202, then heartbeat delta wiring, then master API.

decnet/lifecycle/events.py

@@ -0,0 +1,43 @@
+"""Bus emit helper for DeckyLifecycle transitions.
+
+DB is the source of truth (wizard polls ``GET /deckies/lifecycle?ids=``).
+The bus is best-effort live notification — publish failures are logged
+and swallowed via ``publish_safely``, never propagated.
+"""
+from __future__ import annotations
+
+from typing import Optional
+
+from decnet.bus import topics as _topics
+from decnet.bus.base import BaseBus
+from decnet.bus.publish import publish_safely
+
+
+async def emit_lifecycle(
+    bus: BaseBus | None,
+    *,
+    lifecycle_id: str,
+    decky_name: str,
+    operation: str,
+    status: str,
+    error: Optional[str] = None,
+) -> None:
+    """Publish ``decky.<name>.lifecycle`` with the current transition.
+
+    Payload keys: ``lifecycle_id``, ``operation``, ``status`` and
+    optionally ``error``.  Documented in
+    ``wiki-checkout/Service-Bus.md``.
+    """
+    payload: dict = {
+        "lifecycle_id": lifecycle_id,
+        "operation": operation,
+        "status": status,
+    }
+    if error is not None:
+        payload["error"] = error
+    await publish_safely(
+        bus,
+        _topics.decky_lifecycle(decky_name),
+        payload,
+        event_type=_topics.DECKY_LIFECYCLE,
+    )