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/onElementRemoval.md

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+---
+category: Sensors
+---
+
+# onElementRemoval
+
+Fires when the element or any element containing it is removed from the DOM.
+
+## Usage
+
+```vue {13}
+<script setup lang="ts">
+import { onElementRemoval } from '@vueuse/core'
+import { shallowRef, useTemplateRef } from 'vue'
+
+const btnRef = useTemplateRef('btn')
+const btnState = shallowRef(true)
+const removedCount = shallowRef(0)
+
+function btnOnClick() {
+  btnState.value = !btnState.value
+}
+
+onElementRemoval(btnRef, () => ++removedCount.value)
+</script>
+
+<template>
+  <button
+    v-if="btnState"
+    @click="btnOnClick"
+  >
+    recreate me
+  </button>
+  <button
+    v-else
+    ref="btnRef"
+    @click="btnOnClick"
+  >
+    remove me
+  </button>
+  <b>removed times: {{ removedCount }}</b>
+</template>
+```
+
+### Callback with Mutation Records
+
+The callback receives an array of `MutationRecord` objects that triggered the removal.
+
+```ts
+import { onElementRemoval } from '@vueuse/core'
+
+onElementRemoval(targetRef, (mutationRecords) => {
+  console.log('Element removed', mutationRecords)
+})
+```
+
+### Return Value
+
+Returns a stop function to stop observing.
+
+```ts
+const stop = onElementRemoval(targetRef, callback)
+
+// Later, stop observing
+stop()
+```
+
+## Type Declarations
+
+```ts
+export interface OnElementRemovalOptions
+  extends
+    ConfigurableWindow,
+    ConfigurableDocumentOrShadowRoot,
+    WatchOptionsBase {}
+/**
+ * Fires when the element or any element containing it is removed.
+ *
+ * @param target
+ * @param callback
+ * @param options
+ */
+export declare function onElementRemoval(
+  target: MaybeElementRef,
+  callback: (mutationRecords: MutationRecord[]) => void,
+  options?: OnElementRemovalOptions,
+): Fn
+```