gitplaza-web has chats for repos. It uses kind 1111 comments right now but I might change this, because it wouldn't work with sub chats. What event kind would you use?
Live demo: https://anki-backlog-chat.netlify.app/npub1hw6amg8p24ne08c9gdq8hhpqx0t0pwanpae9z25crn7m9uy7yarse465gr/nodns-protocol/chat