The git tree is the source code

How far down the rabbit hole do you want to go? I prefer to modify the source I work on with Vim using a directory setup which I have Vim and zsh configured to work with easily via custom scripts, aliases, and commands, at least 2 monitors if possible, and so on. How do I enforce that those I distribute to receive this "preferred form for making modifications to the program"?

The line has to be drawn somewhere. IANAL, but I'd think it'd be close to the form which implies the fewest number of "tools" given a "clean system" and is such that a "reasonable modification" could be made. For example, a standard tarball requires tar, one of the decompression binaries, and an editor (which could likely be assumed since modifications need to be made). These "tools" should probably be generally accessible (though not necessarily free), but that's another definition that would need pinned down.