Preview workflows authenticate with a GitHub App installation token

Preview requests from Slack use a GitHub App installation token instead of the shared tuistit personal access token, which was hitting GitHub’s user-level rate limit and failing to publish progress back to Slack. The Helm chart now wires github_app_id, github_app_installation_id, and github_app_private_key from the existing TUIST_OPS_BOT 1Password item so preview workflow dispatch and polling pull from the app installation’s quota. Operators must add the GitHub App fields to that 1Password item before deployment.