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/scripts/reconcile.sh

@@ -790,43 +790,6 @@ for s in d.get('herdr_sessions', []):
         else:
             _pin_and_verify_resume(s, 'cline', cwd, uuid, degraded=True)
 
-# === drift D: stale UUID (cache 의 artifact 가 사라짐) — 보고만, 변경 없음 ===
-ai = d.get('agent_identities', {}) or {}
-cl = (ai.get('claude') or {})
-if cl.get('session_id'):
-    sid = cl['session_id']
-    if not glob.glob(f"{claude_project_dir}/*/{sid}.jsonl"):
-        drifts.append({'class': 'D', 'name': '(claude identity cache)',
-                       'msg': f"stale UUID in agent_identities.claude.session_id: {sid} (jsonl missing)"})
-ag = (ai.get('agy') or {})
-if ag.get('conversation_id'):
-    cid = ag['conversation_id']
-    if not os.path.exists(f"{home}/.gemini/antigravity-cli/conversations/{cid}.db"):
-        drifts.append({'class': 'D', 'name': '(agy identity cache)',
-                       'msg': f"stale UUID in agent_identities.agy.conversation_id: {cid} (.db missing)"})
-hr = (ai.get('hermes') or {})
-if hr.get('session_id'):
-    sid = hr['session_id']
-    hdb = f"{home}/.hermes/state.db"
-    has_session = False
-    if os.path.exists(hdb):
-        try:
-            conn = sqlite3.connect(hdb)
-            r = conn.execute("SELECT 1 FROM sessions WHERE id=?", (sid,)).fetchone()
-            conn.close()
-            has_session = r is not None
-        except Exception:
-            pass
-    if not has_session:
-        drifts.append({'class': 'D', 'name': '(hermes identity cache)',
-                       'msg': f"stale UUID in agent_identities.hermes.session_id: {sid} (session missing from db)"})
-cn = (ai.get('cline') or {})
-if cn.get('session_id'):
-    sid = cn['session_id']
-    if not os.path.exists(f"{home}/.cline/data/sessions/{sid}/{sid}.json"):
-        drifts.append({'class': 'D', 'name': '(cline identity cache)',
-                       'msg': f"stale UUID in agent_identities.cline.session_id: {sid} (session file missing)"})
-
 result = {
     'timestamp': now_iso,
     'yaml_path': yaml_path,