Running a multi-command like `rbt review` with no subcommand crashed with a TypeError. The subparsers were not marked as required, so argparse accepted the empty command line and left `options.args` as None. Multi-commands now print their help text and exit with status 1 when run without any arguments, similar to `git`. The subparsers are also marked as required, so other invalid invocations (such as `rbt review --foo`) show a usage error instead of crashing. They use a `<subcommand>` metavar to keep the error and help output readable. Testing Done: - Ran unit tests. - Ran `rbt review` and saw useful help output instead of a crash.