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/useTimestamp.md

@@ -0,0 +1,93 @@
+---
+category: Animation
+---
+
+# useTimestamp
+
+Reactive current timestamp
+
+## Usage
+
+```ts
+import { useTimestamp } from '@vueuse/core'
+
+const timestamp = useTimestamp({ offset: 0 })
+```
+
+```ts
+import { useTimestamp } from '@vueuse/core'
+// ---cut---
+const { timestamp, pause, resume } = useTimestamp({ controls: true })
+```
+
+## Component Usage
+
+```vue
+<template>
+  <UseTimestamp v-slot="{ timestamp, pause, resume }">
+    Current Time: {{ timestamp }}
+    <button @click="pause()">
+      Pause
+    </button>
+    <button @click="resume()">
+      Resume
+    </button>
+  </UseTimestamp>
+</template>
+```
+
+## Type Declarations
+
+```ts
+export interface UseTimestampOptions<
+  Controls extends boolean,
+> extends ConfigurableScheduler {
+  /**
+   * Expose more controls
+   *
+   * @default false
+   */
+  controls?: Controls
+  /**
+   * Offset value adding to the value
+   *
+   * @default 0
+   */
+  offset?: number
+  /**
+   * Update the timestamp immediately
+   *
+   * @deprecated Please use `scheduler` option instead
+   * @default true
+   */
+  immediate?: boolean
+  /**
+   * Update interval, or use requestAnimationFrame
+   *
+   * @deprecated Please use `scheduler` option instead
+   * @default requestAnimationFrame
+   */
+  interval?: "requestAnimationFrame" | number
+  /**
+   * Callback on each update
+   */
+  callback?: (timestamp: number) => void
+}
+export type UseTimestampReturn<Controls extends boolean> = Controls extends true
+  ? {
+      timestamp: ShallowRef<number>
+    } & Pausable
+  : ShallowRef<number>
+/**
+ * Reactive current timestamp.
+ *
+ * @see https://vueuse.org/useTimestamp
+ * @param options
+ */
+export declare function useTimestamp(
+  options?: UseTimestampOptions<false>,
+): ShallowRef<number>
+export declare function useTimestamp(options: UseTimestampOptions<true>): {
+  timestamp: ShallowRef<number>
+} & Pausable
+```