Moves the four zone-level lifecycle handlers (--copy-zone,
--rename-zone, --remove-zone, --clear-zone-content) out of
main.cpp into a new cli_zone_mgmt.{hpp,cpp} module. All slug-
aware: copy + rename keep file names and manifest mapName in
sync so the result is self-consistent; clear-content empties
the per-feature JSONs without touching terrain.
main.cpp shrinks by 321 lines (4,431 to 4,110).
