Tools growing unexpected code execution is how we keep having problems with secrets and other important things being stolen. If you add this feature to git, generally, then anybody cloning a git repo is going to have to deal with the fact that `git clone` might run arbitrary code. `git clone` is like `cp`. Do you want `cp` to unexpectedly run code? It should never do that.

Why force git to be a build tool?

Just document how to execute the scripts/checks that will be used by ci. Provide a simple script in the repo that folks can intentionally execute.