Designing and implementing robust command-line interfaces using OCaml's cmdliner library, following Daniel Bünzli's design principles. Use when Claude needs to: (1) Design a new CLI or subcommand layout, (2) Implement cmdliner terms and combinators, (3) Enforce clear, predictable, orthogonal options, (4) Produce high-quality --help output and error messages, (5) Integrate cmdliner CLIs into dune-based OCaml projects.