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/useFavicon.md

@@ -0,0 +1,69 @@
+---
+category: Browser
+---
+
+# useFavicon
+
+Reactive favicon
+
+## Usage
+
+```ts {3}
+import { useFavicon } from '@vueuse/core'
+
+const icon = useFavicon()
+
+icon.value = 'dark.png' // change current icon
+```
+
+### Passing a source ref
+
+You can pass a `ref` to it, changes from of the source ref will be reflected to your favicon automatically.
+
+```ts {7}
+import { useFavicon, usePreferredDark } from '@vueuse/core'
+import { computed } from 'vue'
+
+const isDark = usePreferredDark()
+const favicon = computed(() => isDark.value ? 'dark.png' : 'light.png')
+
+useFavicon(favicon)
+```
+
+When a source ref is passed, the return ref will be identical to the source ref
+
+```ts
+import { useFavicon } from '@vueuse/core'
+// ---cut---
+const source = shallowRef('icon.png')
+const icon = useFavicon(source)
+
+console.log(icon === source) // true
+```
+
+## Type Declarations
+
+```ts
+export interface UseFaviconOptions extends ConfigurableDocument {
+  baseUrl?: string
+  rel?: string
+}
+export type UseFaviconReturn =
+  | ComputedRef<string | null | undefined>
+  | Ref<string | null | undefined>
+/**
+ * Reactive favicon.
+ *
+ * @see https://vueuse.org/useFavicon
+ * @param newIcon
+ * @param options
+ */
+export declare function useFavicon(
+  newIcon: ReadonlyRefOrGetter<string | null | undefined>,
+  options?: UseFaviconOptions,
+): ComputedRef<string | null | undefined>
+export declare function useFavicon(
+  newIcon?: MaybeRef<string | null | undefined>,
+  options?: UseFaviconOptions,
+): Ref<string | null | undefined>
+```