Add activity_core/schedule_health: a pure evaluate_schedule_health() verdict
(built on Temporal's num_actions_missed_catchup_window plus a staleness check),
an async check_schedule_health() reader, and post_missed_fire_alert() that emits
a schedule_miss State Hub progress event. Makes a missed fire visible even under
misfire_policy=skip, where Temporal drops it by design. Unit tests for the
verdict logic.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>