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/useFocusWithin.md

@@ -0,0 +1,57 @@
+---
+category: Sensors
+---
+
+# useFocusWithin
+
+Reactive utility to track if an element or one of its decendants has focus. It is meant to match the behavior of the `:focus-within` CSS pseudo-class. A common use case would be on a form element to see if any of its inputs currently have focus.
+
+## Basic Usage
+
+```vue
+<script setup lang="ts">
+import { useFocusWithin } from '@vueuse/core'
+import { ref, watch } from 'vue'
+
+const target = ref()
+const { focused } = useFocusWithin(target)
+
+watch(focused, (focused) => {
+  if (focused)
+    console.log('Target contains the focused element')
+  else
+    console.log('Target does NOT contain the focused element')
+})
+</script>
+
+<template>
+  <form ref="target">
+    <input type="text" placeholder="First Name">
+    <input type="text" placeholder="Last Name">
+    <input type="text" placeholder="Email">
+    <input type="text" placeholder="Password">
+  </form>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UseFocusWithinReturn {
+  /**
+   * True if the element or any of its descendants are focused
+   */
+  focused: ComputedRef<boolean>
+}
+/**
+ * Track if focus is contained within the target element
+ *
+ * @see https://vueuse.org/useFocusWithin
+ * @param target The target element to track
+ * @param options Focus within options
+ */
+export declare function useFocusWithin(
+  target: MaybeElementRef,
+  options?: ConfigurableWindow,
+): UseFocusWithinReturn
+```