Request preview environments from Slack

Operators can now create and delete Tuist preview environments directly from Slack. Posting /preview create or /preview delete in Slack persists the request, posts a status card, and dispatches a GitHub Actions workflow that deploys the preview into the preview Kubernetes cluster with TTL and idle-time cleanup. When Kura caching is enabled, each preview gets its own shared Kura mesh served at kura-<slug>.preview.tuist.dev, and previews run in-cluster Postgres, ClickHouse, and object storage so they behave like staging without sharing managed production data services.