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/common.rs

@@ -326,6 +326,14 @@ client_request_definitions! {
         params: v2::FsCopyParams,
         response: v2::FsCopyResponse,
     },
+    FsWatch => "fs/watch" {
+        params: v2::FsWatchParams,
+        response: v2::FsWatchResponse,
+    },
+    FsUnwatch => "fs/unwatch" {
+        params: v2::FsUnwatchParams,
+        response: v2::FsUnwatchResponse,
+    },
     SkillsConfigWrite => "skills/config/write" {
         params: v2::SkillsConfigWriteParams,
         response: v2::SkillsConfigWriteResponse,
@@ -899,6 +907,7 @@ server_notification_definitions! {
     AccountUpdated => "account/updated" (v2::AccountUpdatedNotification),
     AccountRateLimitsUpdated => "account/rateLimits/updated" (v2::AccountRateLimitsUpdatedNotification),
     AppListUpdated => "app/list/updated" (v2::AppListUpdatedNotification),
+    FsChanged => "fs/changed" (v2::FsChangedNotification),
     ReasoningSummaryTextDelta => "item/reasoning/summaryTextDelta" (v2::ReasoningSummaryTextDeltaNotification),
     ReasoningSummaryPartAdded => "item/reasoning/summaryPartAdded" (v2::ReasoningSummaryPartAddedNotification),
     ReasoningTextDelta => "item/reasoning/textDelta" (v2::ReasoningTextDeltaNotification),
@@ -1478,6 +1487,27 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_fs_watch() -> Result<()> {
+        let request = ClientRequest::FsWatch {
+            request_id: RequestId::Integer(10),
+            params: v2::FsWatchParams {
+                path: absolute_path("tmp/repo/.git"),
+            },
+        };
+        assert_eq!(
+            json!({
+                "method": "fs/watch",
+                "id": 10,
+                "params": {
+                    "path": absolute_path_string("tmp/repo/.git")
+                }
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_list_experimental_features() -> Result<()> {
         let request = ClientRequest::ExperimentalFeatureList {