gapless draft of "Challenges"

Okay! Now I just need to write the summary bullet points (already
started in realtime, if not "Git time", thanks to the Git staging area
and `git add -p`), and do an edit pass, and then I can ask counsel for
political advice on how to publish this thing. (And I need advice,
given how back-and-forth my intuitions are; today I'm not feeling as
hot on the coda-as-LessWrong-comment idea I was talking up in a recent
commit message.)