extensions/apps/marketplaces ownership

Root AGENTS.md also applies. Rules below were moved from the root, not weakened.

Marketplace ownership

Seller protocols, mappings, credentials, inbox logic and Studio UI belong only in extensions/apps/marketplaces/. Generic paid external-order/channel host contracts belong to their existing Core domain owners. Use native app jobs/consent, tenant references, exact currencies, stock, orders, guest projection, receipts and outbox; never implement another marketplace commerce ledger, money calculator or queue. An accepted submission is not a published offer; ambiguous writes must remain fenced. Missing source tax/payment facts require explicit review, never estimates. Run the registered marketplaces provider suite and marketplace UI regressions; see docs/marketplaces.md for current provider boundaries.