0486 — Alert executors are worker definitions the manager claims, never embedded goroutines

Context. The check executors were first hosted as errgroup goroutines inside SystemManager.Run. Hosted that way they were invisible to the fleet, could not be suspended, had no claim arbitration across instances, and made the system manager non-generic — it knew about alerts specifically.

Decision. Each executor is an ordinary worker definition that joins the manager beside janitor, cronscheduler, and waterline; the manager claims it like any other worker.

Consequences. Executors get fleet visibility, suspension, and N-way claim arbitration from the worker machinery, and the system manager stays generic — it hosts worker definitions, not alert code. Rejected: errgroup goroutines inside SystemManager.Run — a parallel hosting mechanism beside the worker machinery that already exists for exactly this.