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/useSubject.md

@@ -0,0 +1,77 @@
+---
+category: '@RxJS'
+---
+
+# useSubject
+
+Bind an RxJS [`Subject`](https://rxjs.dev/guide/subject) to a `ref` and propagate value changes both ways.
+
+## Usage
+
+<!-- TODO: import rxjs error if enable twoslash -->
+
+```ts no-twoslash
+import { useSubject } from '@vueuse/rxjs'
+import { Subject } from 'rxjs'
+
+const subject = new Subject()
+
+// setup()
+const subjectRef = useSubject(subject)
+
+// Changes to subjectRef.value will be pushed to the subject
+subjectRef.value = 'new value'
+
+// Values emitted by the subject will update subjectRef
+subject.next('from subject')
+```
+
+### With BehaviorSubject
+
+When using a `BehaviorSubject`, the returned ref is initialized with the subject's current value and the type does not include `undefined`:
+
+```ts no-twoslash
+import { useSubject } from '@vueuse/rxjs'
+import { BehaviorSubject } from 'rxjs'
+
+const subject = new BehaviorSubject('initial')
+
+// setup()
+const subjectRef = useSubject(subject) // Ref<string>, not Ref<string | undefined>
+console.log(subjectRef.value) // 'initial'
+```
+
+### Error Handling
+
+If you want to add custom error handling to a Subject that might error, you can supply an optional `onError` configuration. Without this, RxJS will treat any error in the supplied observable as an "unhandled error" and it will be thrown in a new call stack and reported to `window.onerror` (or `process.on('error')` if you happen to be in node).
+
+```ts no-twoslash
+import { useSubject } from '@vueuse/rxjs'
+import { Subject } from 'rxjs'
+
+const subject = new Subject()
+
+// setup()
+const subjectRef = useSubject(subject, {
+  onError: (err) => {
+    console.log(err.message) // "oops"
+  },
+},)
+```
+
+## Type Declarations
+
+```ts
+export interface UseSubjectOptions<I = undefined> extends Omit<
+  UseObservableOptions<I>,
+  "initialValue"
+> {}
+export declare function useSubject<H>(
+  subject: BehaviorSubject<H>,
+  options?: UseSubjectOptions,
+): Ref<H>
+export declare function useSubject<H>(
+  subject: Subject<H>,
+  options?: UseSubjectOptions,
+): Ref<H | undefined>
+```