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/useAuth.md

@@ -0,0 +1,123 @@
+---
+category: '@Firebase'
+---
+
+# useAuth
+
+Reactive [Firebase Auth](https://firebase.google.com/docs/auth) binding. It provides a reactive `user` and `isAuthenticated` so you
+can easily react to changes in the users' authentication status.
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useAuth } from '@vueuse/firebase/useAuth'
+import { initializeApp } from 'firebase/app'
+import { getAuth, GoogleAuthProvider, signInWithPopup } from 'firebase/auth'
+
+const app = initializeApp({ /* config */ })
+const auth = getAuth(app)
+const { isAuthenticated, user } = useAuth(auth)
+
+const signIn = () => signInWithPopup(auth, new GoogleAuthProvider())
+</script>
+
+<template>
+  <pre v-if="isAuthenticated">{{ user }}</pre>
+  <div v-else>
+    <button @click="signIn">
+      Sign In with Google
+    </button>
+  </div>
+</template>
+```
+
+## Return Values
+
+| Name              | Type                   | Description                                               |
+| ----------------- | ---------------------- | --------------------------------------------------------- |
+| `user`            | `Ref<User \| null>`    | The current Firebase user, or `null` if not authenticated |
+| `isAuthenticated` | `ComputedRef<boolean>` | Whether a user is currently authenticated                 |
+
+The composable automatically updates when the user's ID token changes (including sign-in, sign-out, and token refresh events) using Firebase's `onIdTokenChanged` listener.
+
+## Type Declarations
+
+```ts
+export interface UseFirebaseAuthOptions {
+  isAuthenticated: ComputedRef<boolean>
+  user: Ref<User | null>
+}
+/**
+ * Reactive Firebase Auth binding
+ *
+ * @see https://vueuse.org/useAuth
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useAuth(auth: Auth): {
+  isAuthenticated: ComputedRef<boolean>
+  user: Ref<
+    {
+      readonly emailVerified: boolean
+      readonly isAnonymous: boolean
+      readonly metadata: {
+        readonly creationTime?: string | undefined
+        readonly lastSignInTime?: string | undefined
+      }
+      readonly providerData: {
+        readonly displayName: string | null
+        readonly email: string | null
+        readonly phoneNumber: string | null
+        readonly photoURL: string | null
+        readonly providerId: string
+        readonly uid: string
+      }[]
+      readonly refreshToken: string
+      readonly tenantId: string | null
+      delete: () => Promise<void>
+      getIdToken: (forceRefresh?: boolean) => Promise<string>
+      getIdTokenResult: (forceRefresh?: boolean) => Promise<IdTokenResult>
+      reload: () => Promise<void>
+      toJSON: () => object
+      readonly displayName: string | null
+      readonly email: string | null
+      readonly phoneNumber: string | null
+      readonly photoURL: string | null
+      readonly providerId: string
+      readonly uid: string
+    } | null,
+    | User
+    | {
+        readonly emailVerified: boolean
+        readonly isAnonymous: boolean
+        readonly metadata: {
+          readonly creationTime?: string | undefined
+          readonly lastSignInTime?: string | undefined
+        }
+        readonly providerData: {
+          readonly displayName: string | null
+          readonly email: string | null
+          readonly phoneNumber: string | null
+          readonly photoURL: string | null
+          readonly providerId: string
+          readonly uid: string
+        }[]
+        readonly refreshToken: string
+        readonly tenantId: string | null
+        delete: () => Promise<void>
+        getIdToken: (forceRefresh?: boolean) => Promise<string>
+        getIdTokenResult: (forceRefresh?: boolean) => Promise<IdTokenResult>
+        reload: () => Promise<void>
+        toJSON: () => object
+        readonly displayName: string | null
+        readonly email: string | null
+        readonly phoneNumber: string | null
+        readonly photoURL: string | null
+        readonly providerId: string
+        readonly uid: string
+      }
+    | null
+  >
+}
+```