fix(skills): enforce allowed-tools metadata (#2626)

* fix(skills): parse allowed-tools frontmatter

* fix(skills): validate allowed-tools metadata

* fix(skills): add shared allowed-tools policy

* fix(subagents): enforce skill allowed-tools

* fix(agent): enforce skill allowed-tools

* refactor(skills): dedupe TypeVar and reuse cached enabled skills

- Drop redundant module-level TypeVar in tool_policy; rely on PEP 695 syntax.
- Expose get_cached_enabled_skills() and have the lead agent reuse it
  instead of synchronously rescanning skills on every request.

* fix(agent): expose config-scoped skill cache

* fix(subagents): pass filtered tools explicitly

* fix(skills): clean allowed-tools policy feedback

backend/tests/test_skills_parser.py

@@ -86,6 +86,33 @@ def test_parse_license_field(tmp_path):
     assert skill.license == "MIT"
 
 
+def test_parse_missing_allowed_tools_returns_none(tmp_path):
+    skill_file = _write_skill(tmp_path, "name: my-skill\ndescription: Test")
+    skill = parse_skill_file(skill_file, category="custom")
+    assert skill is not None
+    assert skill.allowed_tools is None
+
+
+def test_parse_allowed_tools_list(tmp_path):
+    skill_file = _write_skill(tmp_path, 'name: my-skill\ndescription: Test\nallowed-tools: ["bash", "read_file"]')
+    skill = parse_skill_file(skill_file, category="custom")
+    assert skill is not None
+    assert skill.allowed_tools == ["bash", "read_file"]
+
+
+def test_parse_empty_allowed_tools_list(tmp_path):
+    skill_file = _write_skill(tmp_path, "name: my-skill\ndescription: Test\nallowed-tools: []")
+    skill = parse_skill_file(skill_file, category="custom")
+    assert skill is not None
+    assert skill.allowed_tools == []
+
+
+def test_parse_invalid_allowed_tools_returns_none(tmp_path):
+    skill_file = _write_skill(tmp_path, "name: my-skill\ndescription: Test\nallowed-tools: bash")
+    skill = parse_skill_file(skill_file, category="custom")
+    assert skill is None
+
+
 def test_parse_missing_name_returns_none(tmp_path):
     """Skills missing a name field are rejected."""
     skill_file = _write_skill(tmp_path, "description: A test skill")