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/useWindowFocus.md

@@ -0,0 +1,46 @@
+---
+category: Elements
+---
+
+# useWindowFocus
+
+Reactively track window focus with `window.onfocus` and `window.onblur` events.
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useWindowFocus } from '@vueuse/core'
+
+const focused = useWindowFocus()
+</script>
+
+<template>
+  <div>{{ focused }}</div>
+</template>
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseWindowFocus v-slot="{ focused }">
+    Document Focus: {{ focused }}
+  </UseWindowFocus>
+</template>
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Reactively track window focus with `window.onfocus` and `window.onblur`.
+ *
+ * @see https://vueuse.org/useWindowFocus
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useWindowFocus(
+  options?: ConfigurableWindow,
+): ShallowRef<boolean>
+```