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

tests/test_tier1_unit.py

@@ -338,3 +338,67 @@ def test_mqtt_with_retry_failure(mam_sandbox):
     with pytest.raises(ValueError, match="failing"):
         failing_func()
     assert len(calls) == 3
+
+
+def test_b10_no_agent_identities_reader_in_production():
+    """B-10: agent_identities has no writer; no production code may read it."""
+    import pathlib
+    root = pathlib.Path(__file__).resolve().parent.parent
+    targets = [
+        root / ".agents" / "skills" / "lib_py" / "workspace_uuid.py",
+        root / ".agents" / "skills" / "multi-agent-mux-monitor" / "scripts" / "reconcile.sh",
+        root / ".agents" / "skills" / "multi-agent-mux-stop" / "scripts" / "stop_session.sh",
+        root / ".agents" / "skills" / "lib.sh",
+    ]
+    offenders = []
+    for f in targets:
+        for i, line in enumerate(f.read_text().splitlines(), 1):
+            if "agent_identities" not in line:
+                continue
+            if line.lstrip().startswith("#"):     # 금지 규약을 서술하는 주석은 허용
+                continue
+            offenders.append(f"{f.name}:{i}: {line.strip()}")
+    assert not offenders, "agent_identities read path resurrected:\n" + "\n".join(offenders)
+
+
+def test_b10_workspace_uuid_has_no_yaml_import():
+    """B-10: no `import yaml` anywhere in workspace_uuid.py — top-level OR lazy."""
+    import ast, pathlib
+    src = (pathlib.Path(__file__).resolve().parent.parent
+           / ".agents" / "skills" / "lib_py" / "workspace_uuid.py")
+    tree = ast.parse(src.read_text())
+    offenders = []
+    for node in ast.walk(tree):                       # ast.walk → 중첩 깊이 무관
+        if isinstance(node, ast.Import):
+            for a in node.names:
+                if a.name.split(".")[0] == "yaml":
+                    offenders.append(f"line {node.lineno}: import {a.name}")
+        elif isinstance(node, ast.ImportFrom):
+            if (node.module or "").split(".")[0] == "yaml":
+                offenders.append(f"line {node.lineno}: from {node.module} import ...")
+    assert not offenders, "PyYAML dependency reintroduced:\n" + "\n".join(offenders)
+
+
+def test_b10_find_workspace_uuid_runs_without_pyyaml(tmp_path):
+    """B-10: the executed resolution path must not need PyYAML."""
+    import subprocess, sys, os, json, pathlib
+    stub = tmp_path / "noyaml"
+    (stub / "yaml").mkdir(parents=True)
+    (stub / "yaml" / "__init__.py").write_text('raise ImportError("PyYAML absent (stub)")\n')
+    skills = str(pathlib.Path(__file__).resolve().parent.parent / ".agents" / "skills")
+    ws = tmp_path / "ws"; ws.mkdir()
+    env = os.environ.copy()
+    env["PYTHONPATH"] = f"{stub}:{skills}"
+    env["WS_ABS"] = str(ws)
+    env["AGENT"] = "claude"
+    env["MAM_STATE_JSON"] = json.dumps({"herdr_sessions": []})
+    env["YAML_PATH"] = str(tmp_path / "agent-sessions.yaml")
+    env["HOME_DIR"] = str(tmp_path)
+    env["CLAUDE_PROJECT_DIR"] = str(tmp_path / "projects")
+    r = subprocess.run(
+        [sys.executable, "-c",
+         "from lib_py.workspace_uuid import find_workspace_uuid_main; find_workspace_uuid_main()"],
+        capture_output=True, text=True, env=env)
+    assert r.returncode == 0, f"resolution path still needs PyYAML: {r.stderr}"
+    assert "yaml" not in r.stderr.lower(), f"PyYAML touched at runtime: {r.stderr}"
+