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/usePrevious.md

@@ -0,0 +1,40 @@
+---
+category: Utilities
+---
+
+# usePrevious
+
+Holds the previous value of a ref.
+
+## Usage
+
+```ts
+import { usePrevious } from '@vueuse/core'
+import { shallowRef } from 'vue'
+
+const counter = shallowRef('Hello')
+const previous = usePrevious(counter)
+
+console.log(previous.value) // undefined
+
+counter.value = 'World'
+
+console.log(previous.value) // Hello
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Holds the previous value of a ref.
+ *
+ * @see   {@link https://vueuse.org/usePrevious}
+ */
+export declare function usePrevious<T>(
+  value: MaybeRefOrGetter<T>,
+): Readonly<ShallowRef<T | undefined>>
+export declare function usePrevious<T>(
+  value: MaybeRefOrGetter<T>,
+  initialValue: T,
+): Readonly<ShallowRef<T>>
+```