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/useCountdown.md

@@ -0,0 +1,105 @@
+---
+category: Time
+---
+
+# useCountdown
+
+Reactive countdown timer in seconds.
+
+## Usage
+
+```ts
+import { useCountdown } from '@vueuse/core'
+
+const countdownSeconds = 5
+const { remaining, start, stop, pause, resume } = useCountdown(countdownSeconds, {
+  onComplete() {
+
+  },
+  onTick() {
+
+  }
+})
+```
+
+You can use a `ref` to change the initial countdown.
+`start()` and `resume()` also accept a new countdown value for the next countdown.
+
+```ts
+import { useCountdown } from '@vueuse/core'
+import { shallowRef } from 'vue'
+
+const countdown = shallowRef(5)
+const { start, reset } = useCountdown(countdown, {
+})
+
+// change the countdown value
+countdown.value = 10
+
+// start a new countdown with 2 seconds
+start(2)
+
+// reset the countdown to 4, but do not start it
+reset(4)
+
+// start the countdown with the current value of `countdown`
+start()
+```
+
+## Type Declarations
+
+```ts
+export interface UseCountdownOptions extends ConfigurableScheduler {
+  /**
+   *  Interval for the countdown in milliseconds. Default is 1000ms.
+   *
+   * @deprecated Please use `scheduler` option instead
+   */
+  interval?: MaybeRefOrGetter<number>
+  /**
+   * Callback function called when the countdown reaches 0.
+   */
+  onComplete?: () => void
+  /**
+   * Callback function called on each tick of the countdown.
+   */
+  onTick?: () => void
+  /**
+   * Start the countdown immediately
+   *
+   * @deprecated Please use `scheduler` option instead
+   * @default false
+   */
+  immediate?: boolean
+}
+export interface UseCountdownReturn extends Pausable {
+  /**
+   * Current countdown value.
+   */
+  remaining: ShallowRef<number>
+  /**
+   * Resets the countdown and repeatsLeft to their initial values.
+   */
+  reset: (countdown?: MaybeRefOrGetter<number>) => void
+  /**
+   * Stops the countdown and resets its state.
+   */
+  stop: () => void
+  /**
+   * Reset the countdown and start it again.
+   */
+  start: (countdown?: MaybeRefOrGetter<number>) => void
+}
+/**
+ * Reactive countdown timer in seconds.
+ *
+ * @param initialCountdown
+ * @param options
+ *
+ * @see https://vueuse.org/useCountdown
+ */
+export declare function useCountdown(
+  initialCountdown: MaybeRefOrGetter<number>,
+  options?: UseCountdownOptions,
+): UseCountdownReturn
+```