No way to discover available discounts before applying codes

[hemanth]

Problem

The discount extension lets platforms submit codes and receive applied/rejected results. But there's no way to ask "what discounts are available right now?" before the buyer types in a code.

An agent can't tell the buyer "there's a 15% off sale running" unless the agent already knows the code. The business knows about its active promotions, but the protocol has no channel for surfacing them.

What's missing

• A query operation to list active, applicable discounts for a given cart/checkout context
• Distinction between public promotions (anyone can see) and private codes (buyer must know)
• Automatic discount surfacing -- if the business has a "free shipping over $50" rule, the agent should know about it when the cart total crosses $50

Suggestion

Add an optional available_discounts array to cart and checkout responses:

{
  "available_discounts": [
    {
      "id": "disc_summer",
      "title": "Summer sale -- 15% off",
      "type": "automatic",
      "conditions": "Applied automatically to orders over $25",
      "applied": true
    },
    {
      "id": "disc_freeship",
      "title": "Free shipping",
      "type": "automatic",
      "conditions": "Orders over $50",
      "applied": false,
      "remaining_amount": 1200
    }
  ]
}

• type: automatic -- business applies it when conditions are met, no code needed
• type: code -- buyer needs to submit a code (the code itself is NOT exposed, just the existence of the promotion)
• remaining_amount -- how much more the buyer needs to spend to qualify (useful for "add $12 more for free shipping" prompts)

This is business-optional. Merchants that don't want to expose promotions just omit the field.