Or better cp -a AGENTS.md CLAUDE.md (or ln -s AGENTS.md CLAUDE.md if some of your tooling doesn't play well with hardlinks but does with symlinks) then if one changes so does the other as they are the same file with two links.

Not sure how source control will deal with this though, even if it commits to git/other fine Windows users might have trouble (NTFS supports both hardlinks and symlinks, since Vista IIRC, but mklink requires elevated privileges).