docs: add advanced skills for Vitest, Pinia, and Vue built-ins

Add comprehensive reference documentation for:
- Vitest: environments, projects/workspaces, type testing, vi utilities
- Pinia: HMR, Nuxt integration, SSR setup
- Vue: built-in components (Transition, Teleport, Suspense, KeepAlive) and advanced directives

.agents/skills/vueuse-functions/references/extendRef.md

@@ -0,0 +1,76 @@
+---
+category: Reactivity
+---
+
+# extendRef
+
+Add extra attributes to Ref.
+
+## Usage
+
+> Please note the extra attribute will not be accessible in Vue's template.
+
+```ts
+import { extendRef } from '@vueuse/core'
+import { shallowRef } from 'vue'
+
+const myRef = shallowRef('content')
+
+const extended = extendRef(myRef, { foo: 'extra data' })
+
+extended.value === 'content'
+extended.foo === 'extra data'
+```
+
+Refs will be unwrapped and be reactive
+
+```ts
+import { extendRef } from '@vueuse/core'
+// ---cut---
+const myRef = shallowRef('content')
+const extraRef = shallowRef('extra')
+
+const extended = extendRef(myRef, { extra: extraRef })
+
+extended.value === 'content'
+extended.extra === 'extra'
+
+extended.extra = 'new data' // will trigger update
+extraRef.value === 'new data'
+```
+
+## Type Declarations
+
+```ts
+export type ExtendRefReturn<T = any> = Ref<T>
+export interface ExtendRefOptions<Unwrap extends boolean = boolean> {
+  /**
+   * Is the extends properties enumerable
+   *
+   * @default false
+   */
+  enumerable?: boolean
+  /**
+   * Unwrap for Ref properties
+   *
+   * @default true
+   */
+  unwrap?: Unwrap
+}
+/**
+ * Overload 1: Unwrap set to false
+ */
+export declare function extendRef<
+  R extends Ref<any>,
+  Extend extends object,
+  Options extends ExtendRefOptions<false>,
+>(ref: R, extend: Extend, options?: Options): ShallowUnwrapRef<Extend> & R
+/**
+ * Overload 2: Unwrap unset or set to true
+ */
+export declare function extendRef<
+  R extends Ref<any>,
+  Extend extends object,
+  Options extends ExtendRefOptions,
+>(ref: R, extend: Extend, options?: Options): Extend & R
+```