Update Section Map

Workflow

After writer confirms review proposals:
- [ ] Step 1: Snapshot current section-map.md to ops/curator/snapshots/YYYY-MM-DD-section-map.md
- [ ] Step 2: Apply confirmed changes: add / rename / merge / retire / reassign
- [ ] Step 3: Sort sections by `Established` date (oldest first for stable ordering)
- [ ] Step 4: Validate schema (every post in exactly one section or unassigned; every section has promise; retired sections preserved)
- [ ] Step 5: Write new section-map.md
- [ ] Step 6: Update the last_updated timestamp + changelog entry

Schema validation rules