feat: figure out fully remote IO

[nlebovits]

Currently, the IO stuff assumes either you read remote and write the output locally or take a local file and write it to a remote branch. Reads assume there must be a local outpath, and writes (I think) assume the input is local, or at least require a local tempfile. Figure out what functionality currently exists and how to eventually be able to read a remote file, modify it, and write it to a new remote location (or overwrite the original remote file).