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/useIntersectionObserver.md

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+---
+category: Elements
+---
+
+# useIntersectionObserver
+
+Detects that a target element's visibility.
+
+## Usage
+
+```vue
+<script setup lang="ts">
+import { useIntersectionObserver } from '@vueuse/core'
+import { shallowRef, useTemplateRef } from 'vue'
+
+const target = useTemplateRef('target')
+const targetIsVisible = shallowRef(false)
+
+const { stop } = useIntersectionObserver(
+  target,
+  ([entry], observerElement) => {
+    targetIsVisible.value = entry?.isIntersecting || false
+  },
+)
+</script>
+
+<template>
+  <div ref="target">
+    <h1>Hello world</h1>
+  </div>
+</template>
+```
+
+## Directive Usage
+
+```vue
+<script setup lang="ts">
+import { vIntersectionObserver } from '@vueuse/components'
+import { shallowRef, useTemplateRef } from 'vue'
+
+const root = useTemplateRef('root')
+
+const isVisible = shallowRef(false)
+
+function onIntersectionObserver([entry]: IntersectionObserverEntry[]) {
+  isVisible.value = entry?.isIntersecting || false
+}
+</script>
+
+<template>
+  <div>
+    <p>
+      Scroll me down!
+    </p>
+    <div v-intersection-observer="onIntersectionObserver">
+      <p>Hello world!</p>
+    </div>
+  </div>
+
+  <!-- with options -->
+  <div ref="root">
+    <p>
+      Scroll me down!
+    </p>
+    <div v-intersection-observer="[onIntersectionObserver, { root }]">
+      <p>Hello world!</p>
+    </div>
+  </div>
+</template>
+```
+
+[IntersectionObserver MDN](https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver/IntersectionObserver)
+
+## Type Declarations
+
+```ts
+export interface UseIntersectionObserverOptions extends ConfigurableWindow {
+  /**
+   * Start the IntersectionObserver immediately on creation
+   *
+   * @default true
+   */
+  immediate?: boolean
+  /**
+   * The Element or Document whose bounds are used as the bounding box when testing for intersection.
+   */
+  root?: MaybeComputedElementRef | Document
+  /**
+   * A string which specifies a set of offsets to add to the root's bounding_box when calculating intersections.
+   */
+  rootMargin?: MaybeRefOrGetter<string>
+  /**
+   * Either a single number or an array of numbers between 0.0 and 1.
+   * @default 0
+   */
+  threshold?: number | number[]
+}
+export interface UseIntersectionObserverReturn extends Supportable, Pausable {
+  stop: () => void
+}
+/**
+ * Detects that a target element's visibility.
+ *
+ * @see https://vueuse.org/useIntersectionObserver
+ * @param target
+ * @param callback
+ * @param options
+ */
+export declare function useIntersectionObserver(
+  target:
+    | MaybeComputedElementRef
+    | MaybeRefOrGetter<MaybeElement[]>
+    | MaybeComputedElementRef[],
+  callback: IntersectionObserverCallback,
+  options?: UseIntersectionObserverOptions,
+): UseIntersectionObserverReturn
+```