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/usePageLeave.md

@@ -0,0 +1,43 @@
+---
+category: Sensors
+---
+
+# usePageLeave
+
+Reactive state to show whether the mouse leaves the page.
+
+## Usage
+
+```ts
+import { usePageLeave } from '@vueuse/core'
+
+const isLeft = usePageLeave()
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UsePageLeave v-slot="{ isLeft }">
+    Has Left Page: {{ isLeft }}
+  </UsePageLeave>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UsePageLeaveOptions extends ConfigurableWindow {}
+export type UsePageLeaveReturn = ShallowRef<boolean>
+/**
+ * Reactive state to show whether mouse leaves the page.
+ *
+ * @see https://vueuse.org/usePageLeave
+ * @param options
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function usePageLeave(
+  options?: UsePageLeaveOptions,
+): UsePageLeaveReturn
+```