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/usePerformanceObserver.md

@@ -0,0 +1,48 @@
+---
+category: Browser
+---
+
+# usePerformanceObserver
+
+Observe performance metrics.
+
+## Usage
+
+```ts
+import { usePerformanceObserver } from '@vueuse/core'
+
+const entrys = ref<PerformanceEntry[]>([])
+usePerformanceObserver({
+  entryTypes: ['paint'],
+}, (list) => {
+  entrys.value = list.getEntries()
+})
+```
+
+## Type Declarations
+
+```ts
+export type UsePerformanceObserverOptions = PerformanceObserverInit &
+  ConfigurableWindow & {
+    /**
+     * Start the observer immediate.
+     *
+     * @default true
+     */
+    immediate?: boolean
+  }
+/**
+ * Observe performance metrics.
+ *
+ * @see https://vueuse.org/usePerformanceObserver
+ * @param options
+ */
+export declare function usePerformanceObserver(
+  options: UsePerformanceObserverOptions,
+  callback: PerformanceObserverCallback,
+): {
+  isSupported: UseSupportedReturn
+  start: () => void
+  stop: () => void
+}
+```