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/useActiveElement.md

@@ -0,0 +1,86 @@
+---
+category: Elements
+---
+
+# useActiveElement
+
+Reactive `document.activeElement`. Returns a shallow ref that updates when focus changes.
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useActiveElement } from '@vueuse/core'
+import { watch } from 'vue'
+
+const activeElement = useActiveElement()
+
+watch(activeElement, (el) => {
+  console.log('focus changed to', el)
+})
+</script>
+```
+
+### Shadow DOM Support
+
+By default, `useActiveElement` will traverse into shadow DOM to find the deeply active element. Set `deep: false` to disable this behavior.
+
+```ts
+import { useActiveElement } from '@vueuse/core'
+
+// Only get the shadow host, not the element inside shadow DOM
+const activeElement = useActiveElement({ deep: false })
+```
+
+### Track Element Removal
+
+Set `triggerOnRemoval: true` to update the active element when the currently active element is removed from the DOM. This uses a `MutationObserver` under the hood.
+
+```ts
+import { useActiveElement } from '@vueuse/core'
+
+const activeElement = useActiveElement({ triggerOnRemoval: true })
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseActiveElement v-slot="{ element }">
+    Active element is {{ element?.dataset.id }}
+  </UseActiveElement>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UseActiveElementOptions
+  extends ConfigurableWindow, ConfigurableDocumentOrShadowRoot {
+  /**
+   * Search active element deeply inside shadow dom
+   *
+   * @default true
+   */
+  deep?: boolean
+  /**
+   * Track active element when it's removed from the DOM
+   * Using a MutationObserver under the hood
+   * @default false
+   */
+  triggerOnRemoval?: boolean
+}
+export type UseActiveElementReturn<T extends HTMLElement = HTMLElement> =
+  ShallowRef<T | null | undefined>
+/**
+ * Reactive `document.activeElement`
+ *
+ * @see https://vueuse.org/useActiveElement
+ * @param options
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useActiveElement<T extends HTMLElement>(
+  options?: UseActiveElementOptions,
+): UseActiveElementReturn<T>
+```