Add emitLastMessage flag to agent.continue()

When calling continue() with emitLastMessage=true, the agent loop
emits message_start/message_end events for the last message in context.
This allows messages added outside the loop (e.g., hook messages via
sendHookMessage) to trigger proper TUI rendering.

Changes across packages:
- packages/ai: agentLoopContinue() accepts emitLastMessage parameter
- packages/agent: Agent.continue(), transports updated to pass flag
- packages/coding-agent: sendHookMessage passes true when triggerTurn

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

@@ -602,8 +602,8 @@ export class AgentSession {
 				message.display,
 				message.details,
 			);
-			// Start a new turn - agent.continue() works because last message is user role
-			await this.agent.continue();
+			// Start a new turn - emit message events for the hook message so TUI can render it
+			await this.agent.continue(true);
 		} else {
 			// Just append to agent state and session, no turn
 			this.agent.appendMessage(appMessage);