### Summary
Add the v2 app-server filesystem watch RPCs and notifications, wire them
through the message processor, and implement connection-scoped watches
with notify-backed change delivery. This also updates the schema
fixtures, app-server documentation, and the v2 integration coverage for
watch and unwatch behavior.
This allows clients to efficiently watch for filesystem updates, e.g. to
react on branch changes.
### Testing
- exercise watch lifecycles for directory changes, atomic file
replacement, missing-file targets, and unwatch cleanup
Add a protocol-level filesystem surface to the v2 app-server so Codex
clients can read and write files, inspect directories, and subscribe to
path changes without relying on host-specific helpers.
High-level changes:
- define the new v2 fs/readFile, fs/writeFile, fs/createDirectory,
fs/getMetadata, fs/readDirectory, fs/remove, fs/copy RPCs
- implement the app-server handlers, including absolute-path validation,
base64 file payloads, recursive copy/remove semantics
- document the API, regenerate protocol schemas/types, and add
end-to-end tests for filesystem operations, copy edge cases
Testing plan:
- validate protocol serialization and generated schema output for the
new fs request, response, and notification types
- run app-server integration coverage for file and directory CRUD paths,
metadata/readDirectory responses, copy failure modes, and absolute-path
validation
