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/useAbs.md

@@ -0,0 +1,31 @@
+---
+category: '@Math'
+---
+
+# useAbs
+
+Reactive `Math.abs`.
+
+## Usage
+
+```ts
+import { useAbs } from '@vueuse/math'
+
+const value = ref(-23)
+const absValue = useAbs(value) // Ref<23>
+```
+
+## Type Declarations
+
+```ts
+/**
+ * Reactive `Math.abs`.
+ *
+ * @see https://vueuse.org/useAbs
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useAbs(
+  value: MaybeRefOrGetter<number>,
+): ComputedRef<number>
+```