Request and manage previews from Slack

Operators can now create and delete preview environments directly from Slack using /preview create and /preview delete. tuist-ops validates the request, posts a Block Kit status card, and dispatches a GitHub Actions workflow that installs the Tuist chart into the preview cluster. Each preview carries its own embedded Postgres, ClickHouse, and MinIO object storage so it does not share managed production-like services, and expires or idle previews are automatically cleaned up by a sweep workflow that uninstalls the Helm release, deletes the shared Kura instance, and removes the namespace.