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/src/codex_message_processor.rs

@@ -893,7 +893,9 @@ impl CodexMessageProcessor {
             | ClientRequest::FsGetMetadata { .. }
             | ClientRequest::FsReadDirectory { .. }
             | ClientRequest::FsRemove { .. }
-            | ClientRequest::FsCopy { .. } => {
+            | ClientRequest::FsCopy { .. }
+            | ClientRequest::FsWatch { .. }
+            | ClientRequest::FsUnwatch { .. } => {
                 warn!("Filesystem request reached CodexMessageProcessor unexpectedly");
             }
             ClientRequest::ConfigRequirementsRead { .. } => {