Function spawn 

pub fn spawn(
    rx: Receiver<TaskEvent>,
    backend: Arc<dyn ExternalBackend>,
    task_manager: Arc<TaskManager>,
    store: Arc<TaskStore>,
    repo: String,
)

Spawn a task that listens for Done events and unblocks parent tasks.

This mirrors the logic in tick_unblock_parents but triggers instantly instead of waiting for the next tick. The tick-loop call remains as a fallback for any events missed due to subscriber lag.