The https://github.com/github/spec-kit which I had used for a few projects, like e.g. https://github.com/mykrok/mykrok .
I saw specflow mentioned when I was looking for something "smaller" -- just ability to store claude code project files from under project's folder instead of global ~/.claude/.... But it seems that specflow is rather like spec-kit, but would be great to have some systematic summary comparison between the two.