Function print_deploy_id 

pub fn print_deploy_id(deploy_id: &str)

Print a successful deployment’s ID to stdout.

This is the one intentional stdout write in human mode. output() sends the pretty summary to stderr so terminals still show it, while the deploy ID — the machine-useful handoff to wait/promote — goes to stdout on its own line. That split is what lets callers compose:

DEPLOY_ID=$(mz-deploy stage)
mz-deploy stage | xargs mz-deploy wait

In JSON mode we skip this, because the structured result already carries deploy_id on stdout and a bare ID would corrupt the JSON stream. The print_stdout allow is intentional for exactly this reason.