Removed the pending.json file that shadowed observable state. Now
discovers pending spawns directly from POLECAT_STARTED messages in
the Deacon's inbox.
Changes:
- CheckInboxForSpawns: Discovers from mail, no more LoadPending/SavePending
- TriggerPendingSpawns: Archives mail after successful trigger
- PruneStalePending: Archives old messages instead of pruning from JSON
The mail system is now the source of truth for pending spawns.
Closes: hq-i31f7
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>