On-demand preview environments can be requested and removed from Slack

Operators can now request and manage preview environments directly from Slack. A new /preview create command and /preview delete command in tuist-ops persist the request, show a status card in Slack, and dispatch a GitHub Actions workflow that deploys an isolated preview into the preview Kubernetes cluster. Previews include embedded analytics and object storage, and are automatically removed when their TTL expires or they become idle. This is an internal operations feature, not a change visible to Tuist CLI users.