### 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
For each feature we have:
1. Trait exposed on environment
2. **Local Implementation** of the trait
3. Remote implementation that uses the client to proxy via network
4. Handler implementation that handles PRC requests and calls into
**Local Implementation**
The idea is that codex-exec exposes an Environment struct with services
on it. Each of those is a trait.
Depending on construction parameters passed to Environment they are
either backed by local or remote server but core doesn't see these
differences.
Adds an environment crate and environment + file system abstraction.
Environment is a combination of attributes and services specific to
environment the agent is connected to:
File system, process management, OS, default shell.
The goal is to move most of agent logic that assumes environment to work
through the environment abstraction.
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
