Bare operations so that this work with jujutsu, etc.

I'm using jujutsu so please provide commands that does not create any branch, checkout any branch, etc.
For comparison, graphite has `--branch` arg for essential commands so it can work with jujutsu.

I'll even question why it isn't designed like this. Like you only need
`gh stack `: Stack `branch1` on top of `branch2`, or if branch2 is missing, it will be the first branch in the stack
`gh submit `: Push a tracked branch to github.

Start by reviewing the existing command behavior and the jujutsu and Graphite comparison described in the issue. Define which operations must avoid branch creation or checkout, then clarify the proposed command interface and the behavior for a missing second branch and tracked-branch submission.

Written by the indexing model from the issue text.