that's a great point indeed - we have Terragrunt support on the roadmap but didn't manage to ship yet. What kind of stuff if you don't mind sharing?
Really what a codegen would want to do in this case would be to make a terragrunt.hcl file that would make the accompanying terraform files callable.
It really wouldn't nessesitiate anything else unless you want to make it easy to parameterize values per CI environment. Then you want multiple terragrunt.hcls