Delegate Mergify admin powers to non-GitHub Owners

A new Roles tab in Settings lets GitHub Owners delegate Mergify admin powers to individual users without making them GitHub Owners. Each user can hold any combination of four roles: Billing Admin (subscription and billing), CI Admin (CI Insights configuration and ci-scoped application keys), Default Product Action Admin (GitHub integrations and product enablement), and Delegation Admin (managing the roster itself).

This is useful when a finance teammate needs billing access but shouldn’t run merges, or when a CI engineer needs to tune fleet settings without becoming a GitHub Owner.