Previews use GitHub App tokens for workflow calls

Tuist Previews now dispatches and polls GitHub Actions preview workflow runs using a short-lived GitHub App installation token instead of a long-lived personal token. This fixes failures caused by hitting the shared user-level GitHub rate limit and removes the dependency on a single bot user’s credentials for preview operations. Deployments that include the tuist-ops chart need the GitHub App identifier, installation identifier, and private key configured in the existing TUIST_OPS_BOT secret before preview requests are issued.