Moves the three item-export handlers (--export-zone-items-md,
--export-project-items-md, --export-project-items-csv) out of
main.cpp into a new cli_items_export.{hpp,cpp} module. All
three render items.json data as human-readable Markdown / CSV
reports for design docs, PR descriptions, GitHub Pages, and
spreadsheet pivot-table workflows.
main.cpp shrinks by 262 lines (5,349 to 5,087).
