Function app_exit 

pub async fn app_exit(state: &AppState) -> usize

App exit (main.rs, after the servers stop): tear down every live agent with Policy::app_exit(), concurrently, and wait for every close already in flight, all under APP_EXIT_CAP. Then stop any Shell() session left over (one whose agent is already gone, or whose teardown hit the cap). Returns how many controllers it closed.