gh-pr-reply

The user wants to post a reply to a specific PR review thread, identified by 6-char prefix from /gh-pr-comments output.

Extract the prefix and the reply body from the user's message. If the user has not yet run /gh-pr-comments <N> this session, instruct them to run it first so the prefix cache is populated. Then run /gh-pr-reply <prefix> "<body>". Output only the runner's one-line summary.