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/useSupported.md

@@ -0,0 +1,29 @@
+---
+category: Utilities
+---
+
+# useSupported
+
+SSR compatibility `isSupported`
+
+## Usage
+
+```ts
+import { useSupported } from '@vueuse/core'
+
+const isSupported = useSupported(() => navigator && 'getBattery' in navigator)
+
+if (isSupported.value) {
+  // do something
+  navigator.getBattery
+}
+```
+
+## Type Declarations
+
+```ts
+export type UseSupportedReturn = ComputedRef<boolean>
+export declare function useSupported(
+  callback: () => unknown,
+): UseSupportedReturn
+```