startLifecycleWatcher

Public, idempotent entry point to start the wildcard lifecycle watcher.

The watcher is normally started lazily by submitRun / respondInput because for fresh (self-created) threads the SSE stream would 404 if opened before the server has the thread row. Callers that already know the thread exists server-side (StreamController.hydrate of an existing thread) can use this to start the watcher up front. The watcher subscribes to wildcard lifecycle events across every namespace, so it sees arbitrarily- nested subagent lifecycle messages that the narrow root content pump (running at depth: 1) wouldn't reach — that's what makes subagent discovery work for historical thread loads.

Idempotent — repeat calls reuse the in-flight start promise.