Allow custom model selection for subagents

What variant of Codex are you using?

cli

What feature would you like to see?

Description
Hi team! While looking into the subagent implementation, I noticed that the model choice appears to be hardcoded to gpt-5.1-codex-mini medium.

Use Case
I would like to switch the subagent model to gpt-5.3-codex-spark for my specific use case to leverage its capabilities.

Questions / Expected Behavior

Is there currently an existing way (e.g., via environment variables or config files) to override the default subagent model?

If not, could we expose this as a configurable parameter in a future update?

I'd be happy to submit a PR to help make this configurable if it aligns with the project's roadmap. Thanks!

Additional information

_No response_