find_subcommand_or_default : CliConfig, List(Str) -> { config : CliConfig, subcommand_path : List(Str) }
Walks the subcommand tree from the root CLI config and returns the target command config. If the path is invalid, it falls back to the deepest command its longest valid prefix names, and returns that prefix as the path so the help header always names the command whose content is shown.