Promotes 'Offer' from a prefilled DM to a real, queryable event:
- Buyer publishes a 30403 offer (d/a/p + optional amount + message)
- Seller sees incoming offers on the listing detail page and can Accept/Decline
(publishes a 30403 response referencing the offer)
- Buyer sees their offer's status and can Withdraw it
- New: useMarketplaceOffers (parse + hooks), OfferDialog, ListingOffersPanel
- NIP.md documents kind 30403; AGENTS.md updated
Gate: 101 tests, 0 errors/warnings, boot smoke green.
