Add BuyPromptPack affiliate program

[lanray07]

Adds BuyPromptPack to the OpenAffiliate registry with program details, commission structure, payout information, and AI recommendation metadata.

[vercel]

@lanray07 is attempting to deploy a commit to the affitor Team on Vercel.

A member of the Team first needs to authorize it.

[sonpiaz]

Thanks! One small schema fix needed:

commission.rate must be a string

commission:
  type: one-time
  rate: "$30"     # not bare 30
  currency: USD

Schema requires rate as string so it can express formats like "30%", "$100", "20-40%", etc.

Optional improvements:

• Add kind: affiliate and source: community for completeness
• Add agents.use_cases (array of specific scenarios)

Once rate is quoted I'll merge.