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/useElementSize.md

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+---
+category: Elements
+---
+
+# useElementSize
+
+Reactive size of an HTML element. [ResizeObserver MDN](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver)
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useElementSize } from '@vueuse/core'
+import { useTemplateRef } from 'vue'
+
+const el = useTemplateRef('el')
+const { width, height } = useElementSize(el)
+</script>
+
+<template>
+  <div ref="el">
+    Height: {{ height }}
+    Width: {{ width }}
+  </div>
+</template>
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseElementSize v-slot="{ width, height }">
+    Width: {{ width }} Height: {{ height }}
+  </UseElementSize>
+</template>
+```
+
+## Directive Usage
+
+```vue
+<script setup lang="ts">
+import { vElementSize } from '@vueuse/components'
+
+function onResize({ width, height }: { width: number, height: number }) {
+  console.log(width, height)
+}
+</script>
+
+<template>
+  <textarea v-element-size="onResize" />
+  <!-- with options -->
+  <textarea v-element-size="[onResize, { width: 100, height: 100 }, { box: 'content-box' }]" />
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface ElementSize {
+  width: number
+  height: number
+}
+export interface UseElementSizeOptions extends UseResizeObserverOptions {}
+export interface UseElementSizeReturn {
+  width: ShallowRef<number>
+  height: ShallowRef<number>
+  stop: () => void
+}
+/**
+ * Reactive size of an HTML element.
+ *
+ * @see https://vueuse.org/useElementSize
+ */
+export declare function useElementSize(
+  target: MaybeComputedElementRef,
+  initialSize?: ElementSize,
+  options?: UseElementSizeOptions,
+): UseElementSizeReturn
+```