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/isDefined.md

@@ -0,0 +1,31 @@
+---
+category: Utilities
+---
+
+# isDefined
+
+Non-nullish checking type guard for Ref.
+
+## Usage
+
+```ts
+import { isDefined } from '@vueuse/core'
+
+const example = ref(Math.random() ? 'example' : undefined) // Ref<string | undefined>
+
+if (isDefined(example))
+  example // Ref<string>
+```
+
+## Type Declarations
+
+```ts
+export type IsDefinedReturn = boolean
+export declare function isDefined<T>(
+  v: ComputedRef<T>,
+): v is ComputedRef<Exclude<T, null | undefined>>
+export declare function isDefined<T>(
+  v: Ref<T>,
+): v is Ref<Exclude<T, null | undefined>>
+export declare function isDefined<T>(v: T): v is Exclude<T, null | undefined>
+```