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-stop/scripts/stop_session.sh

@@ -164,7 +164,7 @@ if herdr has-session -t "$SESSION_NAME" 2>/dev/null; then
 fi
 
 # 캡처: kill 직전에 conversation id 를 해결 (process/jsonl 이 아직 살아있을 때).
-# find_workspace_uuid 가 tier-1(row) -> tier-2(workspace-scoped disk scan) -> tier-3(cache)
+# find_workspace_uuid 가 tier-1(row) -> tier-2(workspace-scoped disk scan)
 # 를 알아서 시도하므로 herdr 생사와 무관하게 동작.
 CAPTURED_UUID=""
 if [ "$CAPTURE_ID" = "1" ] && [ -n "$TARGET_CWD" ]; then
@@ -284,12 +284,6 @@ if purge and purge_uuid:
         for item in adapter.purge_artifacts(purge_uuid, ctx):
             print(f"purged: {item}", flush=True)
         target[adapter.own_key] = None
-    # agent_identities 는 cache — 이 워크스페이스 것일 때만 비운다
-    ai = (d.get('agent_identities') or {}).get(agent) or {}
-    if ai.get('project_cwd') == ws:
-        if adapter and (ai.get('session_id') == purge_uuid or ai.get('conversation_id') == purge_uuid):
-            for field in adapter.identity_cache_fields:
-                ai[field] = None
 elif purge and not purge_uuid:
     print("WARN: --purge-conversation requested but no workspace-scoped UUID resolved; nothing purged", flush=True)