fix(skills): block category path traversal in skill manager (#3844)

Validate category names in _create_skill() before using them as
filesystem path segments. Previously, categories like '../escape' or
'/tmp/pwned' could write skill files outside ~/.hermes/skills/.

Adds _validate_category() that rejects slashes, backslashes, absolute
paths, and non-alphanumeric characters (reuses existing VALID_NAME_RE).

Tests: 5 new tests for traversal, absolute paths, and valid categories.

Salvaged from PR #1939 by Gutslabs.

tests/tools/test_skill_manager_tool.py

@@ -6,6 +6,7 @@ from unittest.mock import patch
 
 from tools.skill_manager_tool import (
     _validate_name,
+    _validate_category,
     _validate_frontmatter,
     _validate_file_path,
     _find_skill,
@@ -82,6 +83,22 @@ class TestValidateName:
         assert "Invalid skill name 'skill@name'" in err
 
 
+class TestValidateCategory:
+    def test_valid_categories(self):
+        assert _validate_category(None) is None
+        assert _validate_category("") is None
+        assert _validate_category("devops") is None
+        assert _validate_category("mlops-v2") is None
+
+    def test_path_traversal_rejected(self):
+        err = _validate_category("../escape")
+        assert "Invalid category '../escape'" in err
+
+    def test_absolute_path_rejected(self):
+        err = _validate_category("/tmp/escape")
+        assert "Invalid category '/tmp/escape'" in err
+
+
 # ---------------------------------------------------------------------------
 # _validate_frontmatter
 # ---------------------------------------------------------------------------
@@ -191,6 +208,29 @@ class TestCreateSkill:
             result = _create_skill("my-skill", "no frontmatter here")
         assert result["success"] is False
 
+    def test_create_rejects_category_traversal(self, tmp_path):
+        skills_dir = tmp_path / "skills"
+        skills_dir.mkdir()
+
+        with patch("tools.skill_manager_tool.SKILLS_DIR", skills_dir):
+            result = _create_skill("my-skill", VALID_SKILL_CONTENT, category="../escape")
+
+        assert result["success"] is False
+        assert "Invalid category '../escape'" in result["error"]
+        assert not (tmp_path / "escape").exists()
+
+    def test_create_rejects_absolute_category(self, tmp_path):
+        skills_dir = tmp_path / "skills"
+        skills_dir.mkdir()
+        outside = tmp_path / "outside"
+
+        with patch("tools.skill_manager_tool.SKILLS_DIR", skills_dir):
+            result = _create_skill("my-skill", VALID_SKILL_CONTENT, category=str(outside))
+
+        assert result["success"] is False
+        assert f"Invalid category '{outside}'" in result["error"]
+        assert not (outside / "my-skill" / "SKILL.md").exists()
+
 
 class TestEditSkill:
     def test_edit_existing_skill(self, tmp_path):