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/lib_py/verify_session.py

@@ -7,7 +7,7 @@ def mam_orchestrator_uuids():
     global _MAM_ORC_CACHE
     if _MAM_ORC_CACHE is not None:
         return _MAM_ORC_CACHE
-    import os, sys, json, sqlite3, yaml
+    import os, sys, json, sqlite3
     override = os.environ.get("MAM_ORCHESTRATOR_UUIDS")
     if override is not None:
         if not override.strip():
@@ -47,6 +47,7 @@ def mam_orchestrator_uuids():
                     pass
             if (d_obj is None or "orchestrator_uuids" not in d_obj) and os.path.exists(yaml_p):
                 try:
+                    import yaml
                     with open(yaml_p) as f:
                         d_obj = yaml.safe_load(f) or {}
                 except Exception: