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/test/compaction-hooks.test.ts

@@ -64,7 +64,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			resolvedPath: "/test/test-hook.ts",
 			handlers,
 			customMessageRenderers: new Map(),
-			setSendHandler: () => {},
+			commands: new Map(),
+			setSendMessageHandler: () => {},
+			setAppendEntryHandler: () => {},
 		};
 	}
 
@@ -240,7 +242,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				],
 			]),
 			customMessageRenderers: new Map(),
-			setSendHandler: () => {},
+			commands: new Map(),
+			setSendMessageHandler: () => {},
+			setAppendEntryHandler: () => {},
 		};
 
 		createSession([throwingHook]);
@@ -284,7 +288,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				],
 			]),
 			customMessageRenderers: new Map(),
-			setSendHandler: () => {},
+			commands: new Map(),
+			setSendMessageHandler: () => {},
+			setAppendEntryHandler: () => {},
 		};
 
 		const hook2: LoadedHook = {
@@ -307,7 +313,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				],
 			]),
 			customMessageRenderers: new Map(),
-			setSendHandler: () => {},
+			commands: new Map(),
+			setSendMessageHandler: () => {},
+			setAppendEntryHandler: () => {},
 		};
 
 		createSession([hook1, hook2]);