How can I incorporate agile development into CS1/CS2 courses?
Don't. 

Agile is great, but it's mostly about how to manage the development process, interact between the different parties involved, communicate status and break down larger projects of work into achievable segments. When you're trying to understand what a loop/function/class/object/etc is there's really no need to also think about the software engineering issues for larger projects, teams, etc. Students only have capacity to learn so much and I'd argue it's counterproductive to have them spend some of their attention on learning agile. 

By all means talk about it if you're looking at software engineering practices, but it shouldn't be a focus.

Test driven development might be ok to teach, although I suspect it will just complicate things for new students.