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/useFps.md

@@ -0,0 +1,28 @@
+---
+category: Sensors
+---
+
+# useFps
+
+Reactive FPS (frames per second).
+
+## Usage
+
+```ts
+import { useFps } from '@vueuse/core'
+
+const fps = useFps()
+```
+
+## Type Declarations
+
+```ts
+export interface UseFpsOptions {
+  /**
+   * Calculate the FPS on every x frames.
+   * @default 10
+   */
+  every?: number
+}
+export declare function useFps(options?: UseFpsOptions): ShallowRef<number>
+```