Addresses apple/container#141, where containers don't receive filesystem
events on mounted volumes, preventing incremental rebuilds and other
file-watching features. This PR implements the guest-side components for
FSNotify. Host-side implementation in the container repo will complete
the pipeline.
Summary:
- Add gRPC protocol definitions for filesystem event notifications
- Implement guest-side event handler that generates Linux inotify events
- Add CLI testing tool (`cctl fsnotify`) and integration test
infrastructure