Set group credits limits

Map synced directory groups to per-member credits limits (up to 100 per request). Identify each group by its stable WorkOS group id; renaming a group in your identity provider never changes the id, so mappings survive renames. Pass null for limit to unmap a group (members fall back to the global limit). Enforcement is read-time: members pick a mapping change up at their next request, with no per-user writes.