A community based topic aggregation platform built on atproto

chore: retire the build-loop trackers; fold durable backlog into PRD §12 master

The author-owned-posts loop is complete (8 tasks, merged). plan.md and loop_state.md were the loop's throwaway coordination artifacts (512e00e precedent); durable follow-ups now live in PRD §11 (deploy runbook) + §12 (backlog). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>