Worth documenting that PR jobs always run on the branch tip, not on the speculative merge

[sunshowers]

By default, GitHub Actions CI runs PR jobs on a speculative merge commit between the tip of the PR branch and the target branch as of the time the PR was last updated. But buildomat runs CI against the PR tip.

On the GHA side, this is hinted at in https://github.com/actions/checkout/?tab=readme-ov-file#checkout-pull-request-head-commit-instead-of-merge-commit but is not as clear as it could be.

I think it's worth documenting this clearly because it's a fairly common source of confusion.