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/watchOnce.md

@@ -0,0 +1,41 @@
+---
+category: Watch
+---
+
+# watchOnce
+
+Shorthand for watching value with `{ once: true }`. Once the callback fires once, the watcher will be stopped.
+See [Vue's docs](https://vuejs.org/guide/essentials/watchers.html#once-watchers) for full details.
+
+## Usage
+
+Similar to `watch`, but with `{ once: true }`
+
+```ts
+import { watchOnce } from '@vueuse/core'
+
+watchOnce(source, () => {
+  // triggers only once
+  console.log('source changed!')
+})
+```
+
+## Type Declarations
+
+```ts
+export declare function watchOnce<T extends Readonly<MultiWatchSources>>(
+  source: [...T],
+  cb: WatchCallback<MapSources<T>, MapOldSources<T, true>>,
+  options?: Omit<WatchOptions<true>, "once">,
+): WatchHandle
+export declare function watchOnce<T>(
+  source: WatchSource<T>,
+  cb: WatchCallback<T, T | undefined>,
+  options?: Omit<WatchOptions<true>, "once">,
+): WatchHandle
+export declare function watchOnce<T extends object>(
+  source: T,
+  cb: WatchCallback<T, T | undefined>,
+  options?: Omit<WatchOptions<true>, "once">,
+): WatchHandle
+```