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/useIntervalFn.md

@@ -0,0 +1,50 @@
+---
+category: Animation
+---
+
+# useIntervalFn
+
+Wrapper for `setInterval` with controls
+
+## Usage
+
+```ts
+import { useIntervalFn } from '@vueuse/core'
+
+const { pause, resume, isActive } = useIntervalFn(() => {
+  /* your function */
+}, 1000)
+```
+
+## Type Declarations
+
+```ts
+export interface UseIntervalFnOptions {
+  /**
+   * Start the timer immediately
+   *
+   * @default true
+   */
+  immediate?: boolean
+  /**
+   * Execute the callback immediately after calling `resume`
+   *
+   * @default false
+   */
+  immediateCallback?: boolean
+}
+export type UseIntervalFnReturn = Pausable
+/**
+ * Wrapper for `setInterval` with controls
+ *
+ * @see https://vueuse.org/useIntervalFn
+ * @param cb
+ * @param interval
+ * @param options
+ */
+export declare function useIntervalFn(
+  cb: Fn,
+  interval?: MaybeRefOrGetter<number>,
+  options?: UseIntervalFnOptions,
+): UseIntervalFnReturn
+```