refactor(uuid): resolve B-10 by deprecating agent_identities and removing PyYAML dependency

- Remove dead agent_identities read path and PyYAML import from workspace_uuid.py
- Defer eager PyYAML import in verify_session.py to lazy YAML fallback branch
- Simplify UUID resolution to 2-tier model (tier-1 own row ID -> tier-2 adapter scan)
- Clean up unused Drift D and ghost cache clearing in reconcile.sh and stop_session.sh
- Add 3 regression guards in tests/test_tier1_unit.py (266/266 PASS)
- Update IMPROVEMENTS.md, VERSIONS.md, and SKILL.md files

.agents/skills/multi-agent-mux-monitor/SKILL.md

@@ -137,16 +137,6 @@ YAML:  no such session
   ```
 - **C-ambiguous. Multiple candidates detected**: If multiple candidate transcripts match an unassigned session, the monitor avoids random pinning, reports `C-ambiguous`, and sets `last_visible_status: "ambiguous: N candidates"`.
 
-### D. Stale UUID (artifact gone)
-
-```
-YAML:  agent_identities.claude.session_id=87dc548e-...
-disk:  ~/.claude/projects/.../87dc548e-...jsonl: missing
-       → report it, but DO NOT delete from YAML
-       (the user may have moved the file or the disk may be temporarily unavailable;
-        only `--purge-conversation` should remove the id)
-```
-
 ## Pitfalls
 
 - **Don't expect `--once` to stay alive** — it does a single pass and exits. Use `--subscribe` for continuous monitoring.