fix(subdirectory_hints): prevent loading AGENTS.md outside workspace

SubdirectoryHintTracker was scanning directories outside the active
working directory, allowing files like ~/.codex/AGENTS.md or
~/.claude/CLAUDE.md to be loaded and injected into the agent context.
This causes cross-agent context contamination and instruction mixup.

Add _is_ancestor_or_same() helper and a path boundary check in
_is_valid_subdir(): only directories within the working directory tree
(i.e. path.is_relative_to(working_dir)) are allowed.

Also add exist_ok=True to mkdir() calls in new tests to prevent
pytest-xdist race conditions when workers share the same tmp_path parent.

Tests added:
- test_outside_working_dir_rejected: verifies sibling dirs are blocked
- test_outside_working_dir_absolute_path_rejected: verifies ~/.codex paths blocked
- test_inside_workspace_subdir_allowed: verifies normal subdir access unaffected
- test_sibling_repo_not_loaded_via_ancestor_walk: ancestor walk stays within workspace

agent/subdirectory_hints.py

@@ -45,6 +45,15 @@ _COMMAND_TOOLS = {"terminal"}
 # Prevents scanning all the way to / for deeply nested paths.
 _MAX_ANCESTOR_WALK = 5
 
+
+def _is_ancestor_or_same(a: Path, b: Path) -> bool:
+    """Check if *a* is the same as or an ancestor of *b* (parent directory check)."""
+    try:
+        b.relative_to(a)
+        return True
+    except ValueError:
+        return False
+
 class SubdirectoryHintTracker:
     """Track which directories the agent visits and load hints on first access.
 
@@ -158,7 +167,13 @@ class SubdirectoryHintTracker:
             self._add_path_candidate(token, candidates)
 
     def _is_valid_subdir(self, path: Path) -> bool:
-        """Check if path is a valid directory to scan for hints."""
+        """Check if path is a valid directory to scan for hints.
+
+        Only allow subdirectories within the working directory tree.
+        This prevents loading AGENTS.md from outside the active workspace
+        (e.g. ~/.codex/AGENTS.md, ~/.claude/CLAUDE.md), which causes
+        cross-agent context contamination and instruction mixup.
+        """
         try:
             if not path.is_dir():
                 return False
@@ -166,12 +181,43 @@ class SubdirectoryHintTracker:
             return False
         if path in self._loaded_dirs:
             return False
+        # Reject paths outside the working directory tree.
+        # path.resolve() may differ from working_dir.resolve() due to symlinks,
+        # but path.is_relative_to(working_dir) handles both absolute and
+        # symlinked paths correctly on Python 3.9+.
+        try:
+            if not path.is_relative_to(self.working_dir):
+                return False
+        except (OSError, ValueError):
+            # Older Python or path resolution error — fall back to parent
+            # check as a best-effort safeguard.
+            if not _is_ancestor_or_same(self.working_dir, path):
+                return False
         return True
 
     def _load_hints_for_directory(self, directory: Path) -> Optional[str]:
-        """Load hint files from a directory. Returns formatted text or None."""
+        """Load hint files from a directory. Returns formatted text or None.
+
+        Only loads hints from directories within the working directory tree.
+        """
         self._loaded_dirs.add(directory)
 
+        # Reject paths outside the working directory tree.
+        try:
+            if not directory.is_relative_to(self.working_dir):
+                logger.debug(
+                    "Skipping hint files in %s — outside working_dir %s",
+                    directory, self.working_dir,
+                )
+                return None
+        except (OSError, ValueError):
+            if not _is_ancestor_or_same(self.working_dir, directory):
+                logger.debug(
+                    "Skipping hint files in %s — outside working_dir %s",
+                    directory, self.working_dir,
+                )
+                return None
+
         found_hints = []
         for filename in _HINT_FILENAMES:
             hint_path = directory / filename