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-status/SKILL.md

@@ -105,7 +105,6 @@ lab-paper-pdf2md-creator-claude              default      running    alive  clau
 | `A` | YAML `running`, herdr dead | session died without going through `multi-agent-mux-stop`. *Could* auto-terminate but won't — that's `multi-agent-mux-monitor`'s job. |
 | `B` | herdr alive, not in YAML | ad-hoc session someone started without `multi-agent-mux-create`. Suggest: "use multi-agent-mux-create to register, or multi-agent-mux-stop to clean up." |
 | `C` | YAML has `claude_session_id_own: null` AND a new *.jsonl exists | new session id materialized; suggest: "run multi-agent-mux-resume or reconcile to register it." |
-| `D` | YAML has UUID in `agent_identities`, but the on-disk artifact is gone | stale UUID; user should `multi-agent-mux-stop --purge-conversation` to clean up. |
 
 ## Pitfalls