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/useSSRWidth.md

@@ -0,0 +1,47 @@
+---
+category: Browser
+---
+
+# useSSRWidth
+
+Used to set a global viewport width which will be used when rendering SSR components that rely on the viewport width like [`useMediaQuery`](../useMediaQuery/index.md) or [`useBreakpoints`](../useBreakpoints/index.md)
+
+## Usage
+
+```ts
+import { provideSSRWidth } from '@vueuse/core'
+
+const app = createApp(App)
+
+provideSSRWidth(500, app)
+```
+
+Or in the root component
+
+```vue
+<script setup lang="ts">
+import { provideSSRWidth } from '@vueuse/core'
+
+provideSSRWidth(500)
+</script>
+```
+
+To retrieve the provided value if you need it in a subcomponent
+
+```vue
+<script setup lang="ts">
+import { useSSRWidth } from '@vueuse/core'
+
+const width = useSSRWidth()
+</script>
+```
+
+## Type Declarations
+
+```ts
+export declare function useSSRWidth(): number | undefined
+export declare function provideSSRWidth(
+  width: number | null,
+  app?: App<unknown>,
+): void
+```