refactor: use Path.is_relative_to() for skill_view boundary check

Replace the string-based startswith + os.sep approach with
Path.is_relative_to() (Python 3.9+, we require 3.10+). This is
the idiomatic pathlib way to check path containment — it handles
separators, case sensitivity, and the equal-path case natively
without string manipulation.

Simplified tests to match: removed the now-unnecessary
test_separator_is_os_native test since is_relative_to doesn't
depend on separator choice.

tools/skills_tool.py

@@ -458,7 +458,7 @@ def skill_view(name: str, file_path: str = None, task_id: str = None) -> str:
             try:
                 resolved = target_file.resolve()
                 skill_dir_resolved = skill_dir.resolve()
-                if not str(resolved).startswith(str(skill_dir_resolved) + os.sep) and resolved != skill_dir_resolved:
+                if not resolved.is_relative_to(skill_dir_resolved):
                     return json.dumps({
                         "success": False,
                         "error": "Path escapes skill directory boundary.",