#8998 Keep one semantic diff comment per pull request

Each push, and above all each force push, added a comment of its own, so a pull request collected copies of its records. The workflow now edits one comment in place to hold every commit, and --since is gone.

"A Person with Claude Opus 5.5" took much of the identity line, above all on a phone. A co-author whose trailer gives an assistant's email is drawn by its short name, keyed on the email since the name changes with the model, and co-authors are joined with "+".

The one comment is edited in place, and GitHub marks an edit only as "edited", so the heading says which commit it is drawn up to and when, and its title links the page as well as "Full page". Each commit shows its author date to the minute in UTC, as the heading does, on the page too.

One comment shows every commit, so after a force push a reader can't tell what changed. With the head before the push, the comment marks each commit that push added (✨, "new") or whose message it edited under the same subject (✏️, "edited"); a rebase that changes no message marks nothing.