PR cost check setup

Built; the public GitHub App listing is not published yet. An organisation admin installs it from PR check settings, on the repositories to check. On every push to a pull request, the app reads the diff, finds model changes (one model id swapped for another) and prices both at your own average tokens per request, at today's price-book version. It does not need a Parezen SDK in the repo.

You set the warn and fail thresholds for cost per request (defaults: warn above +10%, fail above +25%). A small increase passes. A jump that crosses your line warns or fails. Each pull request gets one comment, updated in place on every push, so a busy PR does not fill up with bot comments.

Sample pull request northwind/support-bot #1042. Status warn. Not a live GitHub installation.