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/useOnline.md

@@ -0,0 +1,41 @@
+---
+category: Sensors
+---
+
+# useOnline
+
+Reactive online state. A wrapper of `useNetwork`.
+
+## Usage
+
+```ts
+import { useOnline } from '@vueuse/core'
+
+const online = useOnline()
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseOnline v-slot="{ isOnline }">
+    Is Online: {{ isOnline }}
+  </UseOnline>
+</template>
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Reactive online state.
+ *
+ * @see https://vueuse.org/useOnline
+ * @param options
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useOnline(
+  options?: ConfigurableWindow,
+): Readonly<ShallowRef<boolean>>
+```