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/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,