iter_plugin_dirs stat'd <child>/__init__.py inside every plugin dir, so a
single mode-000 / ACL-denied $HERMES_HOME/plugins/<x> still raised
PermissionError out of the loader, memory-provider discovery (dashboard
memory settings, hermes memory setup, plugins memory picker) and the
user cron-provider scan. Catch OSError per child and log the same
'Skipping unreadable plugin directory' warning the list path already emits.
Part of #111804
