docs: add discovery rules and agent checkout flow

[omsaisudarshan108]

Summary

• Expand the discovery profile example in docs/specification/overview.md with Cart, Loyalty, and Post-Order capabilities.
• Add profile validation rules to clarify required fields and constraints.
• Add an agent checkout reference flow (sequence diagram + minimal payloads) in docs/specification/checkout-rest.md.
• Add scripts/regenerate_artifacts.sh and document it in CONTRIBUTING.md.

Testing

• Not run (docs/script change)

[jingyli]

Thank you for your contribution to the Universal Commerce Protocol!

While we appreciate your enthusiasm for UCP, this PR currently seems to be bundling a few things together. To break it down:

1. Expanding discovery profile example to include additional capabilities/extensions: Given some of the added capabilities/extensions are not UCP-governed constructs, we will decline this part of the change per reasons given in Feat/cart loyalty post order #17. We welcome you to define your own capabilities & extensions, host them on your own domain, and reference them in the profile as long as they follow UCP's governance model.

2. Documentation updates on minimal payload & sequence diagram & new command definition: We will be evaluating the changes and will provide a formal response within 1-2 weeks, but we encourage you to clean up this PR's scope to be specific to those changes to help with our review process.

[omsaisudarshan108]

PR Update - Addressed Maintainer Feedback
Thank you @jingyli for the detailed feedback! I've updated this PR to focus specifically on the documentation improvements as requested.

Changes Made
✅ Removed: Non-UCP-governed capabilities from discovery profile example

Removed dev.ucp.shopping.cart
Removed dev.ucp.shopping.loyalty
Removed dev.ucp.shopping.post_order
The business profile example now only includes UCP-governed capabilities (checkout, fulfillment, discount) following the governance model.

✅ Kept: Documentation improvements

Profile Validation Rules (lines 359-370 in overview.md)

Clear validation requirements for discovery profiles
Version format, service requirements, endpoint constraints
Capability reference validation rules
Agent Checkout Reference Flow (checkout-rest.md)

Mermaid sequence diagram showing canonical REST flow
Minimal payload examples for create and complete operations
Demonstrates the create → update → complete pattern
Developer Tooling

New scripts/regenerate_artifacts.sh convenience script
Updated CONTRIBUTING.md with usage instructions
Summary
The PR scope is now clean and focused on documentation enhancements. All changes follow UCP governance guidelines and are ready for your evaluation.