update-pull-request should honor footer: false

[strawgate]

update-pull-request appears to always append a footer.

It should honor safe-outputs.update-pull-request.footer: false (consistent with other safe outputs).

Expected: when footer: false, no visible footer is added to PR body updates.

As a workaround you can provide a relatively short/blank footer to messages.footer but that's a bit annoying.

[strawgate]

On a very related note: I'm very fond of my update-pr-body Agent which watches PRs for changes that materially impact the PR description and then update the PR description to keep it accurate :)