Solution: Make Github's PR merge and/or squash commit message the PR extended description exactly, prefixed with a permanent link to the PR. Then the trick is

Solution: Make Github's PR merge and/or squash commit message the PR extended description exactly, prefixed with a permanent link to the PR. Then the trick is making your thoughtful well-edited commit messages into a thoughtful well-edited PR description. Problem then is that the initial PR description does not often reflect review discussion. I think it's better to encourage requestors to edit their description than to believe that the person hitting the big "you're done" button will sit and edit a commit message.