agent: Disable project_notifications by default (#34615)

This tool needs more polishing before being generally available.

Release Notes:

- agent: Disabled `project_notifications` tool by default for the time
being

crates/agent/src/thread.rs

@@ -3583,6 +3583,7 @@ fn main() {{
     }
 
     #[gpui::test]
+    #[ignore] // turn this test on when project_notifications tool is re-enabled
     async fn test_stale_buffer_notification(cx: &mut TestAppContext) {
         init_test_settings(cx);