leaving-pr-comments: Attribute Agent Notes

# Leaving PR Comments Prefix every agent-authored comment left on the user's behalf with this GitHub note callout: ```md > [!NOTE] > 🤖 **<model> responding on behalf of <user>** Actual comment starts here. ``` Use the verified public name of the model that wrote the comment, and the user's name or GitHub login. If the harness does not expose a model name, use the harness name. Do not guess. Before posting, check the entire comment, including quoted text and attachments, for secrets and private details. Keep the attribution inside the note and the actual reply outside it. The context line must start with the robot emoji. Do not add a horizontal rule, Setext underline, heading, signature footer, or a second attribution. Apply this format to top-level PR comments, inline review comments, replies to bots or reviewers, and written explanations used before resolving or dismissing feedback. Do not add it to PR descriptions, commit messages, or text the user explicitly asked to post verbatim without attribution. Write the comment itself in a direct style: - Lead with the conclusion. - Cite the relevant code, behavior, check, or reproduction. - Keep it concise and avoid filler, canned thanks, or status narration. - Distinguish verified facts from judgment. Preview the final Markdown before posting so quoting and blank lines render as shown. After posting, verify that GitHub returned the comment or review URL. Resolve a review thread only after its response was posted successfully.