Miro, do you know whether one can configure GitHub to not let others merge
your PRs? Usually only the owner of a PR knows how to meaningfully edit the
squash merge commit message, but if somebody else approves the PR and then
merges one usually ends up with the default message.
Nothing that I am aware of. Gitlab allows a "work in progress" label on your PR and a rule that PR can't be merged unless a certain amount of people approves it but that's it.
I guess the best approach in Github is to either directly specify what you want to happen with your PR (squash/rebase/merge) with what comment or to agree with the maintainers in what way the commits will be merged for everyone (and prepare the commit(s) in the way that it works by default).