What is the most minimal code to work with ConTeXt?

This is the most minimal. It just instructs TeX to stop.

\end

While I'm at it. It's also the most minimal code for plain TeX. For LaTeX, the best I could do is this.

\csname@@end\endcsname

I guess

\catcode`@11 \@@end

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