Request and delete preview environments from Slack

Operators can use a Slack /preview form to spin up and tear down Tuist preview environments directly from Slack. tuist-ops records the request, posts a status card, and dispatches the preview workflow with the chosen PR or commit, TTL, requester, and optional Kura replica count. The preview cluster is provisioned with its own in-cluster Postgres, ClickHouse, and MinIO object storage, plus a preview-scoped Kura mesh with a public kura-<slug>.preview.tuist.dev host when shared caching is enabled. Expired or idle previews are cleaned up by the preview sweep workflow, which uninstalls the Helm release, deletes the shared Kura instance, and removes the preview namespace.