012d50b2b2
Add two-layer retention for beads-native channel messages: 1. On-write cleanup (EnforceChannelRetention): - Called after posting to channel - Deletes oldest messages when count > retainCount 2. Deacon patrol backup (PruneAllChannels): - Scans all channels periodically - Uses 10% buffer to avoid thrashing - Catches edge cases: crashed mid-write, manual insertions Part of gt-xfqh1e.13 (channel retention task). Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>