Enable agent to enter plan mode automatically.

What variant of Codex are you using?

CLI

What feature would you like to see?

The agent can actively enter the plan mode by itself consideration rather than entering plan mode only under human control.
In some complicated skills or specs, we would guide the agent to enter the plan mode as the very begnning huge workflow.
Without this feature, we have to write something like "skill requirement" to make the agent warn if current is not under plan mode and stop working.

Additional information

This feature have latent relationship to the issue: https://github.com/openai/codex/issues/10384

Possible Engineering Implementation

Expose the tool named enter_plan_mode or whatsoever to the agent just like what Claude Code did.