Move hook command execution to AgentSession.prompt()

Hook commands registered via pi.registerCommand() are now handled in
AgentSession.prompt() alongside file-based slash commands. This:

- Removes duplicate tryHandleHookCommand from interactive-mode and rpc-mode
- All modes (interactive, RPC, print) share the same command handling logic
- AgentSession._tryExecuteHookCommand() builds CommandContext using:
  - UI context from hookRunner (set by mode)
  - sessionManager, modelRegistry from AgentSession
  - sendMessage via sendHookMessage
  - exec via exported execCommand
- Handler returning string uses it as prompt, undefined returns early

Also:
- Export execCommand from hooks/runner.ts
- Add getUIContext() and getHasUI() to HookRunner
- Make HookRunner.emitError() public for error reporting

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

@@ -182,10 +182,10 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 
 			case "prompt": {
 				// Don't await - events will stream
+				// Hook commands and file slash commands are handled in session.prompt()
 				session
 					.prompt(command.message, {
 						attachments: command.attachments,
-						expandSlashCommands: false,
 					})
 					.catch((e) => output(error(id, "prompt", e.message)));
 				return success(id, "prompt");