AgentEventSink typedef
AgentEventSink =
FutureOr<void> Function(AgentEvent event)
Sink receiving every event the loop emits (pi's AgentEventSink).
Awaiting the returned future delays the loop, which is how the stateful
Agent folds its listeners into the run's settlement.
Implementation
typedef AgentEventSink = FutureOr<void> Function(AgentEvent event);