Re vim bindings, this tweet[1] by John Nunemaker makes it seem unlikely. (Not actually sure how serious he is, but it's a funny tweet - and I say that as a Vim user.)
Perhaps a satisfactory solution for everyone would be to make "esc" defocus the input field? Github already has a "command mode" by default, and focus on the textbox can serve as the mode switching mechanism. The "s" key would then replace "i" in vim.