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/useSum.md

@@ -0,0 +1,36 @@
+---
+category: '@Math'
+---
+
+# useSum
+
+Get the sum of an array reactively
+
+## Usage
+
+```ts
+import { useSum } from '@vueuse/math'
+
+const array = ref([1, 2, 3, 4])
+const sum = useSum(array) // Ref<10>
+```
+
+```ts
+import { useSum } from '@vueuse/math'
+
+const a = ref(1)
+const b = ref(3)
+
+const sum = useSum(a, b, 2) // Ref<6>
+```
+
+## Type Declarations
+
+```ts
+export declare function useSum(
+  array: MaybeRefOrGetter<MaybeRefOrGetter<number>[]>,
+): ComputedRef<number>
+export declare function useSum(
+  ...args: MaybeRefOrGetter<number>[]
+): ComputedRef<number>
+```