Add ability to prepend arbitrary command output as context for the message (a.k.a. "shell mode")

What feature would you like to see?

Weirdly I haven't found an issue for this yet (but maybe I'm wrong and this is a duplicate, sorry if that is the case), but in Gemini CLI it was very useful to use "!" to switch the input box into a shell. I didn't like the way it worked in Gemini since this was a toggle rather than a one-shot thing, but it enabled an essential workflow: I could use command output to augument my requests with needed context (think git diff commands, rg some specific log lines, making a point about inconsistent folder structure via ls -R, etc.)

I really missed this functionality when I decided to try Codex CLI. The addition would be very welcome! Perhaps there's a roadblock with how sandbox works, but since the command is issued by the user the sandbox shouldn't be concerned with that.

Additional information

_No response_