Fix event bus async error handling, clear pending messages on session switch, improve SDK docs

- event-bus.ts: await async handlers to catch errors properly
- agent-session.ts: clear _pendingNextTurnMessages on newSession/switchSession/branch
- sdk.ts: make eventBus first (required) param for discoverHooks/discoverCustomTools
- docs/sdk.md: document eventBus sharing pattern for hook/tool communication

packages/coding-agent/src/core/agent-session.ts

@@ -865,6 +865,7 @@ export class AgentSession {
 		this.sessionManager.newSession(options);
 		this._steeringMessages = [];
 		this._followUpMessages = [];
+		this._pendingNextTurnMessages = [];
 		this._reconnectToAgent();
 
 		// Emit session_switch event with reason "new" to hooks
@@ -1653,6 +1654,7 @@ export class AgentSession {
 		await this.abort();
 		this._steeringMessages = [];
 		this._followUpMessages = [];
+		this._pendingNextTurnMessages = [];
 
 		// Set new session
 		this.sessionManager.setSessionFile(sessionPath);
@@ -1728,6 +1730,9 @@ export class AgentSession {
 			skipConversationRestore = result?.skipConversationRestore ?? false;
 		}
 
+		// Clear pending messages (bound to old session state)
+		this._pendingNextTurnMessages = [];
+
 		if (!selectedEntry.parentId) {
 			this.sessionManager.newSession();
 		} else {