Hook API: replace send() with sendMessage(), add appendEntry() and registerCommand()

Breaking changes to Hook API:
- pi.send(text, attachments?) replaced with pi.sendMessage(message, triggerTurn?)
  - Creates CustomMessageEntry instead of user messages
  - Properly handles queuing during streaming via agent loop
  - Supports optional turn triggering when idle
- New pi.appendEntry(customType, data?) for hook state persistence
- New pi.registerCommand(name, options) for custom slash commands
- Handler types renamed: SendHandler -> SendMessageHandler, new AppendEntryHandler

Implementation:
- AgentSession.sendHookMessage() handles all three cases:
  - Streaming: queues message with _hookData marker, agent loop processes it
  - Not streaming + triggerTurn: appends to state/session, calls agent.continue()
  - Not streaming + no trigger: appends to state/session only
- message_end handler routes based on _hookData presence to correct persistence
- HookRunner gains getRegisteredCommands() and getCommand() methods

New types: HookMessage<T>, RegisteredCommand, CommandContext

packages/coding-agent/src/modes/print-mode.ts

@@ -39,9 +39,14 @@ export async function runPrintMode(
 		hookRunner.onError((err) => {
 			console.error(`Hook error (${err.hookPath}): ${err.error}`);
 		});
-		// No-op send handler for print mode (single-shot, no async messages)
-		hookRunner.setSendHandler(() => {
-			console.error("Warning: pi.send() is not supported in print mode");
+		// Set up handlers - sendHookMessage handles queuing/direct append as needed
+		hookRunner.setSendMessageHandler((message, triggerTurn) => {
+			session.sendHookMessage(message, triggerTurn).catch((e) => {
+				console.error(`Hook sendMessage failed: ${e instanceof Error ? e.message : String(e)}`);
+			});
+		});
+		hookRunner.setAppendEntryHandler((customType, data) => {
+			session.sessionManager.appendCustomEntry(customType, data);
 		});
 		// Emit session event
 		await hookRunner.emit({