Some minor cleanup things

[bitplane]

Some pedantry changes that I didn't want on the other branch:

• Remove execute bit from everything without a #!, and fix the one file that had a broken shebang
• Make README.md readable in the console, mostly anyway (I use vim and less a lot!)
• rename placeholder.txt to .gitkeep (apparently it's a convention, I figured it should start with a period but news to me)
• get rid of lfs since it's not used anymore, including the wget step from the Colab notebooks