Slack preview card updates when deployment finishes

The Slack card posted when a preview is requested now updates automatically once the GitHub Actions workflow finishes: on success the card shows the live preview URL and the GitHub Actions run link, and on failure the card switches to a failure state. The wiring uses the github_actions_token from TUIST_OPS_BOT (exposed as GITHUB_ACTIONS_TOKEN in tuist-ops) plus a preview_monitor worker that polls GitHub for the dispatched run and reconciles the original Slack message, so operators see the full lifecycle in one Slack thread.