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/refDefault.md

@@ -0,0 +1,36 @@
+---
+category: Reactivity
+---
+
+# refDefault
+
+Apply default value to a ref.
+
+## Usage
+
+```ts
+import { refDefault, useStorage } from '@vueuse/core'
+
+const raw = useStorage('key')
+const state = refDefault(raw, 'default')
+
+raw.value = 'hello'
+console.log(state.value) // hello
+
+raw.value = undefined
+console.log(state.value) // default
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Apply default value to a ref.
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function refDefault<T>(
+  source: Ref<T | undefined | null>,
+  defaultValue: T,
+): Ref<T>
+```