Add bot comment to describe release spec warnings

The tool used for the release workflow (`create-release-branch`) currently shows warnings about potential hazards to the author of each release, but these warnings are only viewable to the release author. Reviewers have no way of knowing which warnings were shown.

We should add a workflow that checks release warnings, and adds them to the release PR as a bot comment.

Start by locating the create-release-branch release workflow and how it produces warnings. Trace the release PR creation path and determine where a workflow can read those warnings and post them for reviewers. Done means the release PR contains a bot comment with the warnings shown to the release author.

Written by the indexing model from the issue text.