I've been thinking about this, too. There seems to be a lot of redundancy between the two. Maybe SpecKit uses AGENTS.md if it exists to inform itself for the constitution, then updates AGENTS.md to reference constitution.md?

In the context of spec-driven development, I've started thinking about AGENTS.md in much the same way we use a README -- kind of the "springboard" or "table of contents" for the target audience (in this case, we'll say other engineers) -- which seems to be working pretty well. I have within it things like "here is where the documentation is, here are the ADRs, here are the designs, here are the essential/common commands to run, and this is how I want you to do things." So, that would mean the constitution would be more of a proper constitution. That is, a high level set of non-negotiables for how things should be done (what SpecKit seems to be going for).