Look, if an agent writes the code and the commit message then adding a Co-authored-by by default is ok. Not even showing it before the commit is made is not, and adding the message when AI was just completing code is not.
The appended message isn't even adding any new information, as in this day and age a vast majority of commits is probably "co-authored" by an LLM.
If I ask Claude to write a commit message, it will inserted a co-author line (and an ad), but I can see it and disapprove, add a counter instruction to CLAUDE.md etc