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/test/compaction-hooks.test.ts

@@ -98,6 +98,7 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 				confirm: async () => false,
 				input: async () => null,
 				notify: () => {},
+				custom: () => ({ close: () => {}, requestRender: () => {} }),
 			},
 			false,
 		);