What is the most minimal code to work with ConTeXt?
This is the most minimal. It just instructs TeX to stop.
While I'm at it. It's also the most minimal code for plain TeX. For LaTeX, the best I could do is this.
is shorter, but it's less minimal since TeX has to execute two commands instead of just one.
\starttext Hello world. \stoptext
or even simpler
Hello world. \bye