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.
According to ConTeXt Garden,
is the minimal working example which produces a page and it is for both LuaTeX/MkIV and LuaMetaTeX/LMTX. It produces a blank PDF with a page number. Yeah, new answer to old question, but it's still shorter than
As Henri points out, ConTeXt will run with an empty input file, but it won't produce any PDF.