feat(agent): add direct harness event listeners

Part of I2, "passive events and watch buffering," in packages/agent/docs/harness-v2.md.

packages/agent/src/harness/events.ts

@@ -34,10 +34,41 @@ export interface WatchHandle<TSnapshot> {
 	unsubscribe(): void;
 }
 
-export class HarnessEventBus {
+export class HarnessEventBus implements Events {
+	private readonly listeners = new Map<HarnessEventType, Set<HarnessEventListener>>();
 	private readonly watchListeners = new Set<(event: HarnessEvent) => void>();
 
+	/**
+	 * Register a listener for future events of one type and return its unsubscribe function.
+	 * Earlier events are not replayed, and no snapshot or event buffer is provided.
+	 */
+	on<TType extends HarnessEventType>(
+		type: TType,
+		listener: HarnessEventListener<HarnessEventOfType<TType>>,
+	): () => void {
+		// Reuse this event type's listener set, or create its first set.
+		const listeners = this.listeners.get(type) ?? new Set<HarnessEventListener>();
+		this.listeners.set(type, listeners);
+
+		// Wrap this event-specific callback so it can be stored as a general HarnessEvent listener.
+		// Keep the wrapper reference so unsubscribe can remove that exact function from the set.
+		const receive: HarnessEventListener = (event) => {
+			if (event.type === type) return listener(event as HarnessEventOfType<TType>);
+		};
+		listeners.add(receive);
+		return () => {
+			listeners.delete(receive);
+			if (listeners.size === 0) this.listeners.delete(type);
+		};
+	}
+
+	/** Publish an event to current event subscriptions and watch subscriptions. */
 	emit(event: HarnessEvent): void {
+		// Deliver only to direct listeners registered for this event type.
+		// Async results are not awaited because emit() is synchronous.
+		for (const listener of this.listeners.get(event.type) ?? []) void listener(event);
+
+		// Deliver every event to each watcher; watch() handles buffering until start().
 		for (const listener of this.watchListeners) listener(event);
 	}
 

packages/agent/test/harness/events.test.ts

@@ -16,6 +16,27 @@ const runEndEvent: RunEndEvent = {
 };
 
 describe("HarnessEventBus", () => {
+	it("delivers matching events to direct listeners and watchers", () => {
+		const events = new HarnessEventBus();
+		const direct: RunStartEvent[] = [];
+		const watchEvents: HarnessEvent[] = [];
+		const off = events.on("run_start", (event) => {
+			direct.push(event);
+		});
+		const watch = events.watch(() => null);
+		watch.start((event) => {
+			watchEvents.push(event);
+		});
+
+		events.emit(runStartEvent);
+		events.emit(runEndEvent);
+		off();
+		events.emit(runStartEvent);
+
+		expect(direct).toEqual([runStartEvent]);
+		expect(watchEvents).toEqual([runStartEvent, runEndEvent, runStartEvent]);
+	});
+
 	it("captures a snapshot without an event gap, then flushes and delivers live events", () => {
 		const events = new HarnessEventBus();
 		const expectedSnapshot = { leafId: null };