Well, every bit as mysterious as to why it stopped working; last night it started going into lean burn again. I had done nothing to "fix" it.
I did change the plugs, but that was after it started working again. While they were worn some, the plugs looked great.
I also found this thread:
DIY: forgotten tune-up mods - Honda Civic Forum
Which shows cleaning the EGR valve and a bunch of other maintenance items I should probably address, but I hadn't gotten to doing any of them.