feat(coding-agent): add event bus for tool/hook communication (#431)

* feat(coding-agent): add event bus for tool/hook communication

Adds pi.events API enabling custom tools and hooks to communicate via
pub/sub. Tools can emit events, hooks can listen. Shared EventBus instance
created per session in createAgentSession().

- EventBus interface with emit() and on() methods
- on() returns unsubscribe function
- Threaded through hook and tool loaders
- Documented in hooks.md and custom-tools.md

* fix(coding-agent): wrap event handlers to catch errors

* docs: note async handler error handling for event bus

* feat(coding-agent): add sendMessage to tools, nextTurn delivery mode

- Custom tools now have pi.sendMessage() for direct agent notifications
- New deliverAs: 'nextTurn' queues messages for next user prompt
- Fix: hooks and tools now share the same eventBus (was isolated before)

* fix(coding-agent): nextTurn delivery should always queue, even when streaming

packages/coding-agent/src/core/event-bus.ts

@@ -0,0 +1,33 @@
+import { EventEmitter } from "node:events";
+
+export interface EventBus {
+	emit(channel: string, data: unknown): void;
+	on(channel: string, handler: (data: unknown) => void): () => void;
+}
+
+export interface EventBusController extends EventBus {
+	clear(): void;
+}
+
+export function createEventBus(): EventBusController {
+	const emitter = new EventEmitter();
+	return {
+		emit: (channel, data) => {
+			emitter.emit(channel, data);
+		},
+		on: (channel, handler) => {
+			const safeHandler = (data: unknown) => {
+				try {
+					handler(data);
+				} catch (err) {
+					console.error(`Event handler error (${channel}):`, err);
+				}
+			};
+			emitter.on(channel, safeHandler);
+			return () => emitter.off(channel, safeHandler);
+		},
+		clear: () => {
+			emitter.removeAllListeners();
+		},
+	};
+}