fix(plugins): skip dunder dirs and contain plugin-scan OSErrors

Walking __pycache__ and other dunder children let an unreadable
directory raise out of plugin discovery and fail unrelated tool
calls. Skip those names and treat iterdir/is_dir OSError as an
empty scan for that node.

Fixes #86996

hermes_cli/plugins_discovery.py

@@ -109,7 +109,17 @@ def scan_directory(
     manifests: List[PluginManifest] = []
     if not path.is_dir():
         return manifests
-    for child in sorted(path.iterdir()):
+    try:
+        children = sorted(path.iterdir())
+    except OSError as exc:
+        logger.warning("Failed to scan plugin directory %s: %s", path, exc)
+        return manifests
+    for child in children:
+        # Cache/dunder dirs (__pycache__, __MACOSX__, …) are never
+        # plugins. Walking them can raise PermissionError and take
+        # down every subsequent tool call (#86996).
+        if child.name.startswith("__") and child.name.endswith("__"):
+            continue
         try:
             if not child.is_dir() or (depth == 0 and skip_names and child.name in skip_names):
                 continue

tests/hermes_cli/test_plugin_scan_dunder_dirs.py

@@ -0,0 +1,56 @@
+"""Plugin directory scans must skip dunder dirs and survive OSError (#86996)."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from hermes_cli.plugins import PluginManager
+
+
+def test_scan_skips_dunder_dirs_and_still_loads_real_plugin(tmp_path: Path):
+    root = tmp_path / "plugins"
+    demo = root / "demo"
+    demo.mkdir(parents=True)
+    (demo / "plugin.yaml").write_text("name: demo\nversion: 0.1.0\ndescription: x\n")
+    (demo / "__pycache__").mkdir()
+    pycache = root / "__pycache__"
+    pycache.mkdir()
+    pycache.chmod(0o000)
+    try:
+        found = PluginManager()._scan_directory(root, "user")
+    finally:
+        pycache.chmod(0o700)
+
+    assert [manifest.name for manifest in found] == ["demo"]
+
+
+def test_scan_survives_iterdir_oserror(tmp_path: Path, monkeypatch):
+    root = tmp_path / "plugins"
+    root.mkdir()
+
+    def _boom(_self):
+        raise PermissionError("unreadable plugin root")
+
+    monkeypatch.setattr(Path, "iterdir", _boom, raising=False)
+    found = PluginManager()._scan_directory(root, "user")
+    assert found == []
+
+
+def test_scan_skips_child_whose_is_dir_raises(tmp_path: Path, monkeypatch):
+    root = tmp_path / "plugins"
+    demo = root / "demo"
+    demo.mkdir(parents=True)
+    (demo / "plugin.yaml").write_text("name: demo\nversion: 0.1.0\ndescription: x\n")
+    spooky = root / "spooky"
+    spooky.mkdir()
+
+    original_is_dir = Path.is_dir
+
+    def _is_dir(self):
+        if self.name == "spooky":
+            raise PermissionError("stat failed")
+        return original_is_dir(self)
+
+    monkeypatch.setattr(Path, "is_dir", _is_dir)
+    found = PluginManager()._scan_directory(root, "user")
+    assert [manifest.name for manifest in found] == ["demo"]