Be able to review against remote branches (`origin/main` vs `main`)

When running `crit` AFTER the plan stage (to review local changes), the changes are against the default branch, not the `origin/main` (which I can fetch with `git fetch`)

<img width="669" height="173" alt="Image" src="https://github.com/user-attachments/assets/884d9ccd-b0c7-4f67-89d2-38dee0bd0720" />

Being able to pick `origin/main` here would be useful