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/modes/rpc/rpc-mode.ts

@@ -118,6 +118,11 @@ export async function runRpcMode(session: AgentSession): Promise<never> {
 				notifyType: type,
 			} as RpcHookUIRequest);
 		},
+
+		custom() {
+			// Custom UI not supported in RPC mode
+			return { close: () => {}, requestRender: () => {} };
+		},
 	});
 
 	// Load entries once for session start events