If you're writing design documentation to /tmp you're not the target audience for this kind of thing
Most of the time all you need is a throwaway md file to compensate for context loss and so you can loop on a spec across compaction. Once you have the spec implemented, there's usually no reason to keep it around. The code becomes the spec.
For hard constraints you have AGENTS.md, or put it in another md file. Why do I need additional software for this?