Designs the UX and contract of a command-line program in any language — argument parsing via a real lib (commander/yargs, click/typer, cobra, clap), meaningful exit codes, the stdout=data / stderr=logs split so the tool pipes cleanly, TTY-aware color/spinners that auto-plain when redirected, a --json machine mode, layered config precedence, signal cleanup, and shell completion. Covers the whole interface contract that makes a CLI scriptable, composable, and safe — not the language-internal logic.
