Repository configuration uses .sinatrarc.json

If your repository has an extensionless .sinatrarc, rename it to .sinatrarc.json without changing its contents and commit the rename on the default branch. Commit the same rename on any task branches you still use: existing PR checkouts read the configuration at their verified head commit. Model choices, test hooks, and sandbox settings stay in the renamed file. The repository configuration page links to .sinatrarc.json for editing or creation.