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/core/hooks/runner.ts

@@ -3,7 +3,7 @@
  */
 
 import { spawn } from "node:child_process";
-import type { LoadedHook, SendHandler } from "./loader.js";
+import type { AppendEntryHandler, LoadedHook, SendMessageHandler } from "./loader.js";
 import type {
 	CustomMessageRenderer,
 	ExecOptions,
@@ -12,6 +12,7 @@ import type {
 	HookEvent,
 	HookEventContext,
 	HookUIContext,
+	RegisteredCommand,
 	SessionEvent,
 	SessionEventResult,
 	ToolCallEvent,
@@ -164,12 +165,22 @@ export class HookRunner {
 	}
 
 	/**
-	 * Set the send handler for all hooks' pi.send().
+	 * Set the send message handler for all hooks' pi.sendMessage().
 	 * Call this when the mode initializes.
 	 */
-	setSendHandler(handler: SendHandler): void {
+	setSendMessageHandler(handler: SendMessageHandler): void {
 		for (const hook of this.hooks) {
-			hook.setSendHandler(handler);
+			hook.setSendMessageHandler(handler);
+		}
+	}
+
+	/**
+	 * Set the append entry handler for all hooks' pi.appendEntry().
+	 * Call this when the mode initializes.
+	 */
+	setAppendEntryHandler(handler: AppendEntryHandler): void {
+		for (const hook of this.hooks) {
+			hook.setAppendEntryHandler(handler);
 		}
 	}
 
@@ -218,6 +229,33 @@ export class HookRunner {
 		return undefined;
 	}
 
+	/**
+	 * Get all registered commands from all hooks.
+	 */
+	getRegisteredCommands(): RegisteredCommand[] {
+		const commands: RegisteredCommand[] = [];
+		for (const hook of this.hooks) {
+			for (const command of hook.commands.values()) {
+				commands.push(command);
+			}
+		}
+		return commands;
+	}
+
+	/**
+	 * Get a registered command by name.
+	 * Returns the first command found across all hooks, or undefined if none.
+	 */
+	getCommand(name: string): RegisteredCommand | undefined {
+		for (const hook of this.hooks) {
+			const command = hook.commands.get(name);
+			if (command) {
+				return command;
+			}
+		}
+		return undefined;
+	}
+
 	/**
 	 * Create the event context for handlers.
 	 */