How do you open this in SugarCube? I downloaded the repo but there's no distinct "main" file, I checked the docs and they all say to do it via VSCode and use Tweego rather than Twine/SugarCube directly
Dude, seriously, you need to be a bit proactive if you intend to mod a game, no one is going to hold your hand and give step by step instructions.
However... you have there a folder called game, where everything is neatly ordered. You have batch files for compiling your modifications in core. You can read the documentation in tweego on website, edit the .bat's to see what exactly they do. To edit the files you should have a pretty good idea how twine works and tweego shortcuts. That's for starters.
Some time ago I tried to mess with this game too. I underestimated the amount of work (and talent) you need to write even a simple scene with credible characters. After about a week of efforts (8-10 hours / day) I gave up. My pawns were doing what I wanted them to do, but they had no soul, no personality. Were just white words on a black screen.
To sum up. If you want to write scenes / encounters, be prepared for a lot of effort. If you just want to mess with game constants and let's say... increase the damage you do with high heels or how much you get for an orchid - that's painfully easy.
Good luck.