doc: Update .github/copilot-instructions.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: WCY-dt

.github/copilot-instructions.md

@@ -141,6 +141,7 @@ npm run format    # Format code with Prettier
 ### Pinia Store Usage
 ```typescript
 import { defineStore } from 'pinia'
+import { ref } from 'vue'
 
 export const useMyStore = defineStore('myStore', () => {
   const state = ref(initialValue)