Moves the items.json read-only inspection handlers out of
main.cpp:
--list-items --info-item
--validate-items --validate-project-items
--info-project-items
Item editing handlers (--add-item, --set-item, --remove-item,
--add-quest-reward-item) stay in main.cpp since they share
state with quest reward editing logic and would need a
broader extraction.
main.cpp drops 13,887 → 13,485 lines (-402). Behavior
verified by re-running --list-items on a non-items zone
(same error message).
