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/useToString.md

@@ -0,0 +1,34 @@
+---
+category: Utilities
+---
+
+# useToString
+
+Reactively convert a ref to string.
+
+## Usage
+
+```ts
+import { useToString } from '@vueuse/core'
+import { shallowRef } from 'vue'
+
+const number = shallowRef(3.14)
+const str = useToString(number)
+
+str.value // '3.14'
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Reactively convert a ref to string.
+ *
+ * @see https://vueuse.org/useToString
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useToString(
+  value: MaybeRefOrGetter<unknown>,
+): ComputedRef<string>
+```