The per-command options lived only in main.rs's doc header, so a packaged
binary had no way to show them. They move into a USAGE constant (one copy,
no drift) that `--help`, `-h`, `help` and `<command> --help` print to stdout,
exiting 0. No command prints the usage to stderr and fails; an unknown
command is named and points to --help.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
