CLI should support "Shell mode" to explicitly invoke input as shell command

[RyanHecht]

Sometimes, you know exactly the command you want to run, and you don't want to waste a model turn to generate the command. A "Shell mode" would allow the CLI to interpret a user's prompt as an explicit shell command to accomplish this.

The convention appears to be !, I suggest we use this for indicating shell mode

[DovieW]

for me the main use for this would be so that the command and its output is passed to the model. otherwise i would just use terminal tabs or tmux

[RyanHecht]

This has been shipped in 0.0.333! https://github.com/github/copilot-cli/releases/tag/v0.0.333