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/useMutationObserver.md

@@ -0,0 +1,60 @@
+---
+category: Elements
+---
+
+# useMutationObserver
+
+Watch for changes being made to the DOM tree. [MutationObserver MDN](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver)
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useMutationObserver } from '@vueuse/core'
+import { ref, useTemplateRef } from 'vue'
+
+const el = useTemplateRef('el')
+const messages = ref([])
+
+useMutationObserver(el, (mutations) => {
+  if (mutations[0])
+    messages.value.push(mutations[0].attributeName)
+}, {
+  attributes: true,
+})
+</script>
+
+<template>
+  <div ref="el">
+    Hello VueUse
+  </div>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UseMutationObserverOptions
+  extends MutationObserverInit, ConfigurableWindow {}
+export interface UseMutationObserverReturn extends Supportable {
+  stop: () => void
+  takeRecords: () => MutationRecord[] | undefined
+}
+/**
+ * Watch for changes being made to the DOM tree.
+ *
+ * @see https://vueuse.org/useMutationObserver
+ * @see https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver MutationObserver MDN
+ * @param target
+ * @param callback
+ * @param options
+ */
+export declare function useMutationObserver(
+  target:
+    | MaybeComputedElementRef
+    | MaybeComputedElementRef[]
+    | MaybeRefOrGetter<MaybeElement[]>,
+  callback: MutationCallback,
+  options?: UseMutationObserverOptions,
+): UseMutationObserverReturn
+```