Recommend Prune

Three cleanup categories

1. Retire: section with <2 posts added in 3 months AND no new cluster signal → candidate for retirement.
2. Merge: two sections whose promises overlap >60% semantically OR whose post-sets have >40% cross-fit → merge candidate.
3. Reassign: posts from audit-drift flagged as genuine-drift → suggest target section or move to unassigned.

Workflow

Per Curator run:
- [ ] Step 1: For each section, check retire conditions
- [ ] Step 2: Cross-check section promises for merge candidates
- [ ] Step 3: Collect reassignment candidates from drift audit
- [ ] Step 4: For each proposal: write reasoning + reasons-to-reject
- [ ] Step 5: Emit three lists (retire / merge / reassign)