fix(agent): a parent directory the process cannot stat is not a crash while locating .git (#8751)

_find_git_root walked cwd's parents with Path.exists(); on hosts where a parent is
mode 700 for another user that raised PermissionError out of prompt construction.
Treat an unreadable ancestor as 'no .git here'.

agent/prompt_builder.py

@@ -96,7 +96,15 @@ def _scan_context_content(content: str, filename: str) -> str:
 def _find_git_root(start: Path) -> Optional[Path]:
     """Nearest ancestor (or *start* itself) containing ``.git``, else None."""
     current = start.resolve()
-    return next((p for p in (current, *current.parents) if (p / ".git").exists()), None)
+    # A parent the process may not stat (locked-down /home on shared hosts) is "no .git here", not a crash.
+    return next((p for p in (current, *current.parents) if _exists_or_denied(p / ".git")), None)
+
+
+def _exists_or_denied(path: Path) -> bool:
+    try:
+        return path.exists()
+    except OSError:
+        return False
 
 
 def _find_hermes_md(cwd: Path) -> Optional[Path]:

tests/agent/test_prompt_builder.py

@@ -586,6 +586,21 @@ class TestFindHermesMd:
 
 
 
+    def test_unreadable_parent_is_treated_as_no_git_root(self, tmp_path, monkeypatch):
+        """A parent the process cannot stat (#8751) must not raise out of prompt construction."""
+        import os as _os
+        project = tmp_path / "locked" / "proj"
+        project.mkdir(parents=True)
+        real_exists = Path.exists
+
+        def _exists(self):
+            if self.parent == tmp_path / "locked" and self.name == ".git":
+                raise PermissionError(13, "Permission denied", str(self))
+            return real_exists(self)
+
+        monkeypatch.setattr(Path, "exists", _exists)
+        assert _find_git_root(project) is None
+
     def test_walks_to_git_root(self, tmp_path):
         (tmp_path / ".git").mkdir()
         (tmp_path / ".hermes.md").write_text("root rules")