Function spawn 

pub fn spawn(
    rx: Receiver<TaskEvent>,
    backend: Arc<dyn ExternalBackend>,
    tmux: Arc<TmuxManager>,
    semaphore: Arc<Semaphore>,
    task_manager: Arc<TaskManager>,
    router: Arc<RwLock<Router>>,
    dispatching: Arc<DashMap<String, String>>,
    store: Arc<TaskStore>,
    repo: String,
)

Spawn a task that listens for NeedsReview events and triggers the review agent.

This is the sole trigger for NeedsReview → InReview transitions. The sync tick no longer has a competing NeedsReview loop (removed in fix for issue #857 — both paths firing simultaneously caused double failure-counter increments).