Enable branch protection on main

[lobsterkatie]

I just boneheadedly forgot the -b when checking out a new branch, didn't notice git's refusal to check out a branch which didn't yet exist, committed to main, and pushed. Whoops.

I'm generally not in favor of locking things down any more than necessary, but disallowing such shenanigans, however accidental, feels like a good saving-us-from-ourselves move.

[mitsuhiko]

Done