fix(skills): skill_view directory file_path + skill_manage categorized name resolution

Two agent-facing errors that recur constantly in optimization audit
logs (thousands of occurrences over five months):

1. skill_view(name, file_path='references') returned a raw
   '[Errno 21] Is a directory' OS error. The local-skill branch gated
   on target_file.exists(); a directory passes exists(), fell through
   to read_text(), and raised. The plugin-skill sibling branch already
   gated on is_file() — this aligns the local branch so a directory
   request gets the same helpful not-found payload with
   available_files listing instead of an OS error.

2. skill_manage rejected categorized names ('category/skill-name')
   with 'not found in active profile'. _find_skill matched only the
   bare directory name, while skill_view's own ambiguity hint tells
   the caller to use exactly the categorized form — every call that
   followed the hint failed. _find_skill now also matches the full
   relative path of the skill dir, giving skill_manage resolution
   parity with skill_view across edit/patch/delete/write_file/
   remove_file.

Both fixes are covered by regression tests that fail on main.

tests/tools/test_skill_manager_tool.py

@@ -19,6 +19,7 @@ from tools.skill_manager_tool import (
     _delete_skill,
     _write_file,
     _remove_file,
+    _find_skill,
     skill_manage,
 )
 from agent.skill_utils import (
@@ -200,6 +201,38 @@ class TestEditSkill:
         assert "Updated description" in content
 
 
+    def test_edit_existing_skill_by_categorized_path(self, tmp_path):
+        """Categorized names (``category/skill``) must resolve in skill_manage.
+
+        skill_view's ambiguity hint explicitly tells the caller to use the
+        full categorized path (``category/skill-name``), and skills_list
+        reports skills with their category. But ``_find_skill`` only matched
+        the bare directory name, so every skill_manage call that followed
+        that hint failed with "not found in active profile" — the agent then
+        retried repeatedly, burning LLM round trips (top recurring audit
+        finding). Resolution parity with skill_view is the fix.
+        """
+        with _skill_dir(tmp_path):
+            _create_skill("my-skill", VALID_SKILL_CONTENT, category="software-development")
+            result = _edit_skill("software-development/my-skill", VALID_SKILL_CONTENT_2)
+        assert result["success"] is True, result.get("error")
+        content = (tmp_path / "software-development" / "my-skill" / "SKILL.md").read_text()
+        assert "Updated description" in content
+
+    def test_find_skill_accepts_categorized_path(self, tmp_path):
+        with _skill_dir(tmp_path):
+            _create_skill("my-skill", VALID_SKILL_CONTENT, category="mlops")
+            found = _find_skill("mlops/my-skill")
+        assert found is not None
+        assert found["path"] == tmp_path / "mlops" / "my-skill"
+
+    def test_find_skill_bare_name_still_resolves_nested_skill(self, tmp_path):
+        with _skill_dir(tmp_path):
+            _create_skill("my-skill", VALID_SKILL_CONTENT, category="mlops")
+            found = _find_skill("my-skill")
+        assert found is not None
+        assert found["path"] == tmp_path / "mlops" / "my-skill"
+
     def test_edit_invalid_content_rejected(self, tmp_path):
         with _skill_dir(tmp_path):
             _create_skill("my-skill", VALID_SKILL_CONTENT)