Add planning mode so Codex can plan before executing

What feature would you like to see?

It would be helpful to have a planning mode that lets Codex develop a step-by-step plan before executing commands. This gives users a preview of the intended actions and an opportunity to adjust the strategy. As they say, planning is half the battle, and giving Codex a moment to think before diving in should improve outcomes and reduce unintended side effects. Plus, this would be good for both the CLI and the IDE extension.

Additional information

_No response_