Mode::None is the initial value of mode_ and means "no subcommand
parsed". execute() only reached it through a catch-all default, which
also hid any future Mode enumerator that someone forgot to dispatch.
Give None its own case (same InvalidArgument error as before) and drop
the default so -Wswitch flags an unhandled mode at compile time. The
enumerator stays: removing it would make an unparsed command silently
default to Init.
Add a CLI test that `yams session` with no subcommand is rejected; the
None path stays a defensive guard because require_subcommand() stops
that case at parse time.
Signed-off-by: trvon <me@trevon.dev>
