Use comment pinning to highlight bot comments

[yrodiere]

I see the GitHub REST API exposes the ability to pin a comment, which makes it appear right below the PR description: https://docs.github.com/en/rest/issues/comments?apiVersion=2026-03-10#pin-an-issue-comment

This might be:

• a way to make our bot's comments more obvious (e.g. the latest check failures)
• a way to add content to the PR description without editing it. Less sure about that, since we can only pin one comment, and obviously a comment means a notification, which we don't always want.