HandleWebhook starts a deployment in a detached goroutine that writes
under the app data directory, which is the tests t.TempDir; the tests
slept 100ms and returned, racing Go automatic TempDir cleanup and
intermittently failing with RemoveAll: directory not empty. The webhook
Service now tracks those goroutines in a sync.WaitGroup and exposes
WaitForDeployments; the tests wait on it instead of sleeping. Production
behavior is unchanged apart from making completion observable.
Model: opus-4-8