Add Agent.prompt(AppMessage) overload for custom message types

Instead of using continue() which validates roles, prompt() now accepts
an AppMessage directly. This allows hook messages with role: 'hookMessage'
to trigger proper agent loop with message events.

- Add overloads: prompt(AppMessage) and prompt(string, attachments?)
- sendHookMessage uses prompt(appMessage) instead of appendMessage+continue

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

@@ -584,10 +584,8 @@ export class AgentSession {
 			// Queue for processing by agent loop
 			await this.agent.queueMessage(appMessage);
 		} else if (triggerTurn) {
-			// Append to agent state and session, then trigger a turn
-			this.agent.appendMessage(appMessage);
-			// Start a new turn - emit message events for the hook message so TUI can render it
-			await this.agent.continue(true);
+			// Send as prompt - agent loop will emit message events
+			await this.agent.prompt(appMessage);
 		} else {
 			// Just append to agent state and session, no turn
 			this.agent.appendMessage(appMessage);

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

@@ -115,7 +115,7 @@ export function messageTransformer(messages: AppMessage[]): Message[] {
 				};
 			}
 			if (isHookAppMessage(m)) {
-				// Convert hook message to user message
+				// Convert hook message to user message for LLM
 				return {
 					role: "user",
 					content: m.content,