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/examples/hooks/file-trigger.ts

@@ -21,7 +21,14 @@ export default function (pi: HookAPI) {
 			try {
 				const content = fs.readFileSync(triggerFile, "utf-8").trim();
 				if (content) {
-					pi.send(`External trigger: ${content}`);
+					pi.sendMessage(
+						{
+							customType: "file-trigger",
+							content: `External trigger: ${content}`,
+							display: true,
+						},
+						true, // triggerTurn - get LLM to respond
+					);
 					fs.writeFileSync(triggerFile, ""); // Clear after reading
 				}
 			} catch {