Add ui.custom() for custom hook components with keyboard focus

- Add custom() to HookUIContext: returns { close, requestRender }
- Component receives keyboard input via handleInput()
- CustomMessageComponent default rendering now limits to 5 lines when collapsed
- Add snake.ts example hook with /snake command

packages/coding-agent/src/core/hooks/runner.ts

@@ -52,6 +52,7 @@ const noOpUIContext: HookUIContext = {
 	confirm: async () => false,
 	input: async () => null,
 	notify: () => {},
+	custom: () => ({ close: () => {}, requestRender: () => {} }),
 };
 
 /**