I would use RFC2822 as the underlying format to store any kind of message (pull request, review comment, issue etc.), and of course when displaying messages use CommonMark to style them. Any metadata goes into headers, and Message-ID/In-Reply-To/References headers can be used to link them all together. Using this well defined format you can then decide how to best store and transport all the messages, maybe in a git repo as well, use email, or whatever else works.
I personally think Gerrit works much better than whatever GitHub et al. have for code reviews. As for CI, I would try to keep that out of it as much as possible; just hooks to start a pipeline and to display the result and decide whether to allow a merge or not.
