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/useArrayFindIndex.md

@@ -0,0 +1,59 @@
+---
+category: Array
+---
+
+# useArrayFindIndex
+
+Reactive `Array.findIndex`
+
+## Usage
+
+### Use with array of multiple refs
+
+```ts
+import { useArrayFindIndex } from '@vueuse/core'
+
+const item1 = ref(0)
+const item2 = ref(2)
+const item3 = ref(4)
+const item4 = ref(6)
+const item5 = ref(8)
+const list = [item1, item2, item3, item4, item5]
+const result = useArrayFindIndex(list, i => i % 2 === 0)
+// result.value: 0
+item1.value = 1
+// result.value: 1
+```
+
+### Use with reactive array
+
+```ts
+import { useArrayFindIndex } from '@vueuse/core'
+
+const list = ref([0, 2, 4, 6, 8])
+const result = useArrayFindIndex(list, i => i % 2 === 0)
+// result.value: 0
+list.value.unshift(-1)
+// result.value: 1
+```
+
+## Type Declarations
+
+```ts
+export type UseArrayFindIndexReturn = ComputedRef<number>
+/**
+ * Reactive `Array.findIndex`
+ *
+ * @see https://vueuse.org/useArrayFindIndex
+ * @param list - the array was called upon.
+ * @param fn - a function to test each element.
+ *
+ * @returns the index of the first element in the array that passes the test. Otherwise, "-1".
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useArrayFindIndex<T>(
+  list: MaybeRefOrGetter<MaybeRefOrGetter<T>[]>,
+  fn: (element: T, index: number, array: MaybeRefOrGetter<T>[]) => unknown,
+): UseArrayFindIndexReturn
+```