feat: allow /diff to specify which git repo dir to diff

What feature would you like to see?

The /diff command should take an optional argument specifying which repo to diff.

Additional information

I usually run Claude Code in my ~/src dir, so it can access and work in multiple repos as needed. When I try to do the same in codex, there's no way to use /diff. That, combined with the lack of visible diffs on each patch command, leaves me completely blind to what Codex is actually doing: all I get are its own explanations of what it's done, not any actual output showing me the actual code it's writing (and sometimes executing).

Are you interested in implementing this feature?

If no one else beats me to it, this might be narrowly scoped enough for me to get codex to write it. I'm not set up to run locally built code yet, though, so it won't be this week.