App.query_one searches the app's default screen (_default), but #chat-log/#health-bar/etc. live on the pushed MainScreen. Every HaivemindApp handler failed with NoMatches at runtime. Hold a reference to the MainScreen instance and query it directly so widget lookup no longer depends on which screen is active.
Upstream haivemind_tui.py started the engine synchronously in HaivemindApp.on_mount while MainScreen mounts asynchronously, causing ProbeStart handler to query #chat-log on the default screen (NoMatches). Move the start_run trigger into MainScreen.on_mount so it runs only once the screen is active.
