Moves the three items.json mutation handlers (--set-item,
--copy-zone-items, --clone-item) out of main.cpp into a new
cli_items_mutate.{hpp,cpp} module. All three operate on the
same {"items": [...]} schema; --set-item supports both
id-lookup ("123") and index-lookup ("#0") with strict
"only changed fields are written" semantics, --copy-zone-items
has replace and --merge modes, --clone-item auto-assigns the
smallest unused id.
main.cpp shrinks by 316 lines (5,087 to 4,771) and finally
drops below 5K. items.json now has its full mutation surface
in one focused module instead of scattered through the
megafile.
