People in government agencies, defence and aerospace companies write code in airgapped environments every day.

Essentials like version control, CI/CD and artifact repositories are readily available, as they always have been. Gitlab, Artifactory etc all run just fine.

The problem you'll face with LLM assistants is they require very substantial hardware. I don't know how well that's going to fit your laptop-in-the-middle-of-nowhere use case, but there are projects floating around.

https://github.com/TabbyML/tabby