And thanks for the responses. Let’s do it this way. To summarize:
Use pull request title and body (first PR comment that is always available and automatically created) for proper commit message used for the change log. This should be formatted in the way described in Commit Message Style
When merging a pull request, copy body text under the automatically inserted title in the “merge commit message”.
The log fetches the pull request title and body directly from Github, which allows fixing it post merge. However the body should still be copied when merging, since it provides better details in the actual Git log and in the GH commit overview.