Config option to start in Plan mode by default

What variant of Codex are you using?

CLI

What feature would you like to see?

I may be missing something, but I could not find a documented way to start Codex in Plan mode by default.

I checked the current docs/code and found:

• plan_mode_reasoning_effort, which seems to control reasoning effort for Plan mode once Plan mode is already active
• TUI startup logic that appears to initialize with the Default collaboration mode
• core/session setup that still appears to default to ModeKind::Default

Because of that, I am unsure whether:

1. this is already supported but undocumented,
2. this is intentionally not supported,
3. or this would be a useful feature request.

My use case:

• I use Plan mode as my normal starting workflow
• manually switching to Plan mode each session is repetitive
• a config-based default would make that workflow much smoother

If this is already possible, could you point me to the correct config key or command-line option?

Thanks.

Additional information

_No response_