docs: add advanced skills for Vitest, Pinia, and Vue built-ins

Add comprehensive reference documentation for:
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- Pinia: HMR, Nuxt integration, SSR setup
- Vue: built-in components (Transition, Teleport, Suspense, KeepAlive) and advanced directives

.agents/skills/vueuse-functions/references/useObservable.md

@@ -0,0 +1,91 @@
+---
+category: '@RxJS'
+---
+
+# useObservable
+
+Use an RxJS [`Observable`](https://rxjs.dev/guide/observable), return a `ref`, and automatically unsubscribe from it when the component is unmounted.
+
+## Usage
+
+<!-- TODO: import rxjs error if enable twoslash -->
+
+```ts no-twoslash
+import { useObservable } from '@vueuse/rxjs'
+import { interval } from 'rxjs'
+import { mapTo, scan, startWith } from 'rxjs/operators'
+
+// setup()
+const count = useObservable(
+  interval(1000).pipe(
+    mapTo(1),
+    startWith(0),
+    scan((total, next) => next + total),
+  ),
+)
+```
+
+### Initial Value
+
+You can provide an initial value that will be used before the Observable emits its first value:
+
+```ts no-twoslash
+import { useObservable } from '@vueuse/rxjs'
+import { interval } from 'rxjs'
+
+const count = useObservable(
+  interval(1000),
+  { initialValue: 0 },
+)
+// count.value is 0 until the first emission
+```
+
+### Error Handling
+
+If you want to add custom error handling to an `Observable` 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 { useObservable } from '@vueuse/rxjs'
+import { interval } from 'rxjs'
+import { map } from 'rxjs/operators'
+
+// setup()
+const count = useObservable(
+  interval(1000).pipe(
+    map((n) => {
+      if (n === 10)
+        throw new Error('oops')
+
+      return n + n
+    }),
+  ),
+  {
+    onError: (err) => {
+      console.log(err.message) // "oops"
+    },
+  },
+)
+```
+
+### Options
+
+| Option         | Type                 | Description                              |
+| -------------- | -------------------- | ---------------------------------------- |
+| `initialValue` | `T`                  | Value to use before the Observable emits |
+| `onError`      | `(err: any) => void` | Error handler for Observable errors      |
+
+## Type Declarations
+
+```ts
+export interface UseObservableOptions<I> {
+  onError?: (err: any) => void
+  /**
+   * The value that should be set if the observable has not emitted.
+   */
+  initialValue?: I | undefined
+}
+export declare function useObservable<H, I = undefined>(
+  observable: Observable<H>,
+  options?: UseObservableOptions<I | undefined>,
+): Readonly<Ref<H | I>>
+```