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/useNavigatorLanguage.md

@@ -0,0 +1,57 @@
+---
+category: Sensors
+---
+
+# useNavigatorLanguage
+
+Reactive [navigator.language](https://developer.mozilla.org/en-US/docs/Web/API/Navigator/language).
+
+## Usage
+
+```ts
+import { useNavigatorLanguage } from '@vueuse/core'
+
+const { language } = useNavigatorLanguage()
+
+watch(language, () => {
+  // Listen to the value changing
+})
+```
+
+## Type Declarations
+
+```ts
+export interface NavigatorLanguageState extends Supportable {
+  /**
+   *
+   * ISO 639-1 standard Language Code
+   *
+   * @info The detected user agent language preference as a language tag
+   * (which is sometimes referred to as a "locale identifier").
+   * This consists of a 2-3 letter base language tag that indicates a
+   * language, optionally followed by additional subtags separated by
+   * '-'. The most common extra information is the country or region
+   * variant (like 'en-US' or 'fr-CA').
+   *
+   *
+   * @see https://www.iso.org/iso-639-language-codes.html
+   * @see https://www.loc.gov/standards/iso639-2/php/code_list.php
+   *
+   */
+  language: ShallowRef<string | undefined>
+}
+export interface UseNavigatorLanguageOptions extends ConfigurableWindow {}
+export type UseNavigatorLanguageReturn = Readonly<NavigatorLanguageState>
+/**
+ *
+ * Reactive useNavigatorLanguage
+ *
+ * Detects the currently selected user language and returns a reactive language
+ * @see https://vueuse.org/useNavigatorLanguage
+ *
+ * @__NO_SIDE_EFFECTS__
+ */
+export declare function useNavigatorLanguage(
+  options?: UseNavigatorLanguageOptions,
+): UseNavigatorLanguageReturn
+```