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/toObserver.md

@@ -0,0 +1,38 @@
+---
+category: '@RxJS'
+---
+
+# toObserver
+
+Sugar function to convert a `ref` into an RxJS [Observer](https://rxjs.dev/guide/observer).
+
+## Usage
+
+<!-- TODO: import rxjs error if enable twoslash -->
+
+```ts no-twoslash
+import { from, fromEvent, toObserver, useSubscription } from '@vueuse/rxjs'
+import { interval } from 'rxjs'
+import { map, mapTo, startWith, takeUntil, withLatestFrom } from 'rxjs/operators'
+import { shallowRef, useTemplateRef } from 'vue'
+
+const count = shallowRef(0)
+const button = useTemplateRef('buttonRef')
+
+useSubscription(
+  interval(1000)
+    .pipe(
+      mapTo(1),
+      takeUntil(fromEvent(button, 'click')),
+      withLatestFrom(from(count).pipe(startWith(0))),
+      map(([curr, total]) => curr + total),
+    )
+    .subscribe(toObserver(count)), // same as ).subscribe(val => (count.value = val))
+)
+```
+
+## Type Declarations
+
+```ts
+export declare function toObserver<T>(value: Ref<T>): NextObserver<T>
+```