feat: add discount at checkout feature

[dmoonat]

Implemented the functionality to apply discount codes during the checkout process in the business agent. This includes backend logic for calculating discounts, agent tool integration, and schema updates.

Key Changes:

• Agent Logic (agent.py):

  • Added apply_discount_code tool function to handle discount requests.

• Store Implementation (store.py):

  • Updated _recalculate_checkout to compute and deduct discount amounts from the subtotal.
  • Implemented apply_discount method which currently accepts the hardcoded code "10OFF" for a 10% discount.

• Configuration & Data:

  • ucp.json: Added the dev.ucp.shopping.discount schema definition.
  • agent_card.json: Added the "Apply Discount" skill

• README.md:

  • Added usage steps for applying the "10OFF" discount code in the sample walkthrough