refactor(mcp_client): migrate L171 _is_allowed to Path.is_relative_to (Phase 8 site 1)

The legacy code used 'try: rp.relative_to(cwd); return True; except ValueError: pass'
to check path containment. Python 3.9+ has Path.is_relative_to() which returns
bool directly, eliminating the silent-swallow try/except entirely.

This is a NON-SLIMING migration: the function's behavior is unchanged (still
returns True/False), the test of path containment is the same, but the
implementation no longer relies on bare except+pass. No logging added, no
silenced error, just a cleaner API.

Audit: mcp_client INTERNAL_SILENT_SWALLOW 3 -> 2.

src/mcp_client.py

@@ -165,17 +165,11 @@ def _is_allowed(path: Path) -> bool:
   # Allow current working directory and subpaths by default if no base_dirs
  cwd = Path.cwd().resolve()
  if not _base_dirs:
-  try:
-   rp.relative_to(cwd)
+  if rp.is_relative_to(cwd):
    return True
-  except ValueError:
-   pass
  for bd in _base_dirs:
-  try:
-   rp.relative_to(bd)
+  if rp.is_relative_to(bd):
    return True
-  except ValueError:
-   continue
  return False
 
 def _resolve_and_check(raw_path: str) -> tuple[Path | None, str]: