feat: apply approved Payment Request intake policy

[ovitrif]

Parent: #712
Source: Pubky marketplace Bitkit team brief
Paykit policy owner: pubky/paykit-rs#148
Counterpart: synonymdev/bitkit-android#1210

Scope

• Consume the incoming Payment Request policy approved by the Paykit team.
• If linked-peer-only intake remains, make the buyer contact prerequisite explicit in the iOS integration and copy.
• If intake expands, integrate only the released Paykit SDK contract and its approved trust and abuse controls.
• Keep the trust boundary in Paykit rather than duplicating it in the app.

Acceptance criteria

• iOS follows the Paykit-owned incoming Payment Request contract.
• Buyer-facing behavior matches the approved prerequisite.
• Any required paykit-rs implementation is owned and released by the Paykit team.
• Android references the same approved contract.

Start gate

• Blocked by chore: decide linked-peer Payment Request intake policy pubky/paykit-rs#148.
• If the decision requires SDK changes, blocked until the Paykit team releases them.

[ovitrif]

pubky/paykit-rs#148 was filed in error and is closed. This is no longer gated on a Paykit policy review.