app-server: add filesystem watch support (#14533)

### 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

codex-rs/app-server-protocol/schema/json/v2/FsUnwatchResponse.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Successful response for `fs/unwatch`.",
+  "title": "FsUnwatchResponse",
+  "type": "object"
+}