Function publish 

pub fn publish(
    state_dir: &Path,
    req: PublishRequest<'_>,
) -> Result<Value, String>

Publish a finished stage: check the hand-off rules, commit the updated record on top of the stage’s work, and push car/mp/<id> to the remote.

Push is non-forcing, so two developers finishing the same stage race at the remote and the second is refused rather than overwriting the first.