fetch/gen magicitems subcommands vendor data/open5e/magicitems.json (237
SRD items) and classify them into a generated registry. magic_items.go
holds the MagicItem struct + Kind enum + an init-time overlay merge where
a hand-authored entry wins on ID collision, mirroring the spell and
bestiary imports. Not yet wired into zone loot or the shop — that
integration is a deliberate follow-up.
