Previously, --help/-h was only checked at the top level before command
dispatch. Running e.g. `cmux new-workspace --help` would execute the
command instead of printing help.
Add per-subcommand help text for all commands that take arguments/flags.
The help check runs before socket connect so it works even when cmux is
not running. Commands without dedicated help (ping, help, list-windows,
etc.) fall through to normal behavior.
eb86606841