Moves the two zone-creation handlers (--scaffold-zone,
--mvp-zone) out of main.cpp into a new cli_zone_create.{hpp,cpp}
module. Both kickstart a new authoring session by generating a
new zone directory under custom_zones/ — empty for scaffold,
populated with one of each content type (creature + object +
quest with objective + reward) for mvp. Each goes terrain +
manifest, then mvp adds the demo content positioned at the
tile center.
main.cpp shrinks by 155 lines (4,771 to 4,616).
