I figured it out! It was my own fault.

Here's what happened. My $PATH already had “/opt” in it, for other binaries. I added the path to the `context` binary (which is a path inside /opt/context) to the end of $PATH. So invoking `context` created confusion because the first found item matching “context” in the $PATH is a directory. I don't entirely understand why that produced this particular result, but it's a very easy problem to fix.

Thank you all so much for helping me out. I wouldn't have arrived at this understanding without your convincing me that the problem's specific to my own machine.

What a relief that I can focus on actually using ConTeXt now.

Thank you again.