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/index.ts

@@ -1,4 +1,11 @@
-export { discoverAndLoadHooks, type LoadedHook, type LoadHooksResult, loadHooks, type SendHandler } from "./loader.js";
+export {
+	type AppendEntryHandler,
+	discoverAndLoadHooks,
+	type LoadedHook,
+	type LoadHooksResult,
+	loadHooks,
+	type SendMessageHandler,
+} from "./loader.js";
 export { type HookErrorListener, HookRunner } from "./runner.js";
 export { wrapToolsWithHooks, wrapToolWithHooks } from "./tool-wrapper.js";
 export type {
@@ -17,9 +24,11 @@ export type {
 	HookEvent,
 	HookEventContext,
 	HookFactory,
+	HookMessage,
 	HookUIContext,
 	LsToolResultEvent,
 	ReadToolResultEvent,
+	RegisteredCommand,
 	SessionEvent,
 	SessionEventResult,
 	ToolCallEvent,