MANUAL RUN: point RustyBuns' codeSplitters mirror at checks and compare with Actions #491

Follow-up to #465 (scope item 6). This needs the live desktop app and the user's GitHub token, so no tooling PR closes it.

1. On the desktop app, open the RustyBuns mirror and set its checks: PUT /api/repos/<owner>/rustybuns/checks/settings {"where": "both", "required": ["typecheck"], "post_statuses": true}, or leave required empty to just watch.
2. Push a branch upstream from its branch page. Within 5 minutes (or press ⌘K → "Fetch GitHub's checks now") the branch shows the Actions jobs (typecheck, apps …) next to codeSplitters' own.
3. Write down where it beats Actions and where it falls short, here.