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/set.md

@@ -0,0 +1,30 @@
+---
+category: Utilities
+---
+
+# set
+
+Shorthand for `ref.value = x`
+
+## Usage
+
+```ts
+import { set } from '@vueuse/core'
+
+const a = ref(0)
+
+set(a, 1)
+
+console.log(a.value) // 1
+```
+
+## Type Declarations
+
+```ts
+export declare function set<T>(ref: Ref<T>, value: T): void
+export declare function set<O extends object, K extends keyof O>(
+  target: O,
+  key: K,
+  value: O[K],
+): void
+```