This npm. Asking this question was a fun little journey.
Right on the npm website, the very first sentence starts with "npm", and they do not capitalize it.
That's a pretty good precedent for not capitalizing it. It certainly looks awkward though, which is why I asked the question to begin with. It doesn't feel right to me to start a sentence that way, and I'm sure other some other people would look at it and see a mistake.
Their own documentation forbids capitalization as well:
Straight from Raquel Vélez, an employee:
... read the whole story at css-tricks.com.
always npm, even if starting the sentence.
(this is a common question we get a lot :-) )