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/src/protocol/v2.rs

@@ -2301,6 +2301,52 @@ pub struct FsCopyParams {
 #[ts(export_to = "v2/")]
 pub struct FsCopyResponse {}
 
+/// Start filesystem watch notifications for an absolute path.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct FsWatchParams {
+    /// Absolute file or directory path to watch.
+    pub path: AbsolutePathBuf,
+}
+
+/// Created watch handle returned by `fs/watch`.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct FsWatchResponse {
+    /// Connection-scoped watch identifier used for `fs/unwatch` and `fs/changed`.
+    pub watch_id: String,
+    /// Canonicalized path associated with the watch.
+    pub path: AbsolutePathBuf,
+}
+
+/// Stop filesystem watch notifications for a prior `fs/watch`.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct FsUnwatchParams {
+    /// Watch identifier returned by `fs/watch`.
+    pub watch_id: String,
+}
+
+/// Successful response for `fs/unwatch`.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct FsUnwatchResponse {}
+
+/// Filesystem watch notification emitted for `fs/watch` subscribers.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct FsChangedNotification {
+    /// Watch identifier returned by `fs/watch`.
+    pub watch_id: String,
+    /// File or directory paths associated with this event.
+    pub changed_paths: Vec<AbsolutePathBuf>,
+}
+
 /// PTY size in character cells for `command/exec` PTY sessions.
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
@@ -6497,6 +6543,33 @@ mod tests {
         assert_eq!(decoded, response);
     }
 
+    #[test]
+    fn fs_changed_notification_round_trips() {
+        let notification = FsChangedNotification {
+            watch_id: "0195ec6b-1d6f-7c2e-8c7a-56f2c4a8b9d1".to_string(),
+            changed_paths: vec![
+                absolute_path("tmp/repo/.git/HEAD"),
+                absolute_path("tmp/repo/.git/FETCH_HEAD"),
+            ],
+        };
+
+        let value = serde_json::to_value(&notification).expect("serialize fs/changed notification");
+        assert_eq!(
+            value,
+            json!({
+                "watchId": "0195ec6b-1d6f-7c2e-8c7a-56f2c4a8b9d1",
+                "changedPaths": [
+                    absolute_path_string("tmp/repo/.git/HEAD"),
+                    absolute_path_string("tmp/repo/.git/FETCH_HEAD"),
+                ],
+            })
+        );
+
+        let decoded = serde_json::from_value::<FsChangedNotification>(value)
+            .expect("deserialize fs/changed notification");
+        assert_eq!(decoded, notification);
+    }
+
     #[test]
     fn command_exec_params_default_optional_streaming_flags() {
         let params = serde_json::from_value::<CommandExecParams>(json!({