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/useArrayJoin.md

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+---
+category: Array
+---
+
+# useArrayJoin
+
+Reactive `Array.join`
+
+## Usage
+
+### Use with array of multiple refs
+
+```ts
+import { useArrayJoin } from '@vueuse/core'
+
+const item1 = ref('foo')
+const item2 = ref(0)
+const item3 = ref({ prop: 'val' })
+const list = [item1, item2, item3]
+const result = useArrayJoin(list)
+// result.value: foo,0,[object Object]
+item1.value = 'bar'
+// result.value: bar,0,[object Object]
+```
+
+### Use with reactive array
+
+```ts
+import { useArrayJoin } from '@vueuse/core'
+
+const list = ref(['string', 0, { prop: 'val' }, false, [1], [[2]], null, undefined, []])
+const result = useArrayJoin(list)
+// result.value: string,0,[object Object],false,1,2,,,
+list.value.push(true)
+// result.value: string,0,[object Object],false,1,2,,,,true
+list.value = [null, 'string', undefined]
+// result.value: ,string,
+```
+
+### Use with reactive separator
+
+```ts
+import { useArrayJoin } from '@vueuse/core'
+
+const list = ref(['string', 0, { prop: 'val' }])
+const separator = ref()
+const result = useArrayJoin(list, separator)
+// result.value: string,0,[object Object]
+separator.value = ''
+// result.value: string0[object Object]
+separator.value = '--'
+// result.value: string--0--[object Object]
+```
+
+## Type Declarations
+
+```ts
+export type UseArrayJoinReturn = ComputedRef<string>
+/**
+ * Reactive `Array.join`
+ *
+ * @see https://vueuse.org/useArrayJoin
+ * @param list - the array was called upon.
+ * @param separator - a string to separate each pair of adjacent elements of the array. If omitted, the array elements are separated with a comma (",").
+ *
+ * @returns a string with all array elements joined. If arr.length is 0, the empty string is returned.
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useArrayJoin(
+  list: MaybeRefOrGetter<MaybeRefOrGetter<any>[]>,
+  separator?: MaybeRefOrGetter<string>,
+): UseArrayJoinReturn
+```