[codex] Clarify client-handled preferences and client attribution

[ericfj2140]

Summary:

• Reframes merchant preference verification as automatic client behavior rather than manual buyer behavior.
• Adds a Client Attribution section recommending NIP-89 client tags on marketplace events.
• Clarifies that client tags improve UX/provenance but are not authoritative for validity or authorization.

Rationale:

• Users should not have to manually inspect Nostr events to follow merchant preferences.
• Client attribution helps wallets, marketplace clients, and service-assisted flows provide better UX, debugging, and deep-linking.
• This preserves the existing NIP-89 merchant recommendation model while adding non-consensus attribution guidance.