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/custom-tools/loader.ts

@@ -90,6 +90,7 @@ function createNoOpUIContext(): HookUIContext {
 		confirm: async () => false,
 		input: async () => null,
 		notify: () => {},
+		custom: () => ({ close: () => {}, requestRender: () => {} }),
 	};
 }