If someone can write instructions to download a malicious script into an codebase, hoping an AI agent will read and follow them, they could just as easily write the same wget command directly into a build script or the source itself (probably more effective). In that way it's a very similar threat to the supply chain attacks we're hopefully already familiar with. So it is a serious issue but not necessarily one we don't know how to deal with. The solutions (auditing all third party code, isolating dev environments) just happen to be hard in practice.