Moves the three add-* coordinate-based append handlers
(--add-object, --add-creature, --add-item) out of main.cpp
into a new cli_add.{hpp,cpp} module. All three append a
single entry to a zone's JSON file with optional positional
args after the required ones. --add-item handles raw
nlohmann::json (no dedicated editor class) and auto-assigns
the smallest unused id when the user passes 0 / nothing.
main.cpp shrinks by 198 lines (5,986 to 5,788).
