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/useDocumentVisibility.md

@@ -0,0 +1,44 @@
+---
+category: Elements
+---
+
+# useDocumentVisibility
+
+Reactively track [`document.visibilityState`](https://developer.mozilla.org/en-US/docs/Web/API/Document/visibilityState)
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useDocumentVisibility } from '@vueuse/core'
+
+const visibility = useDocumentVisibility()
+</script>
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseDocumentVisibility v-slot="{ visibility }">
+    Document Visibility: {{ visibility }}
+  </UseDocumentVisibility>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UseDocumentVisibilityOptions extends ConfigurableDocument {}
+export type UseDocumentVisibilityReturn = ShallowRef<DocumentVisibilityState>
+/**
+ * Reactively track `document.visibilityState`.
+ *
+ * @see https://vueuse.org/useDocumentVisibility
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useDocumentVisibility(
+  options?: UseDocumentVisibilityOptions,
+): UseDocumentVisibilityReturn
+```