When a user passed an unknown subcommand or shortcut (e.g. `lark-cli drive
+bogus`), cobra returned `flag.ErrHelp` for the non-runnable group command,
printed the parent help, and exited 0. AI agents couldn't distinguish a
typo from an intentional help request.
Install a tree-wide guard that attaches a RunE to every group command
without its own Run/RunE. The RunE forwards no-args invocations to help
(preserving prior behavior) and emits a structured unknown_subcommand
ExitError (exit 2) listing available subcommands when args are present.