feat: add intent trace extension

[0xtotaylor]

Enhancement Proposal: Intent Trace Extension

Status	Provisional
Author	@0xtotaylor
RFC	rfcs/intent-trace.md

Summary

This proposal extends POST /checkout-sessions/{id}/cancel to accept an optional intent_trace object, enabling agents to transmit structured data about why a checkout session was abandoned.

Type of change

• Enhancement Proposal (new capability/extension per GOVERNANCE.md)

Proposal Contents

Per the Enhancement Proposal process, the RFC (rfcs/intent-trace.md) includes:

• Summary
• Motivation
• Detailed Design
• Risks
• Test Plan
• Graduation Criteria

Implementation

This PR also includes a complete implementation ready to merge once the proposal reaches Implementable status:

• source/schemas/shopping/intent_trace.json — Extension schema
• docs/specification/intent-trace.md — Specification documentation
• mkdocs.yml — Navigation updates

Checklist

• RFC document follows Enhancement Proposal template
• Implementation aligns with RFC detailed design
• Documentation complete
• Schema validation passes
• mkdocs build --strict passes

Requested Action

Requesting TC review to advance from Provisional → Implementable.

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[yanheChen]

Thank you for your contribution to the Universal Commerce Protocol!

While we appreciate your ethusiam for UCP, this doesn't align with our process for Enhancement proposals (https://github.com/Universal-Commerce-Protocol/ucp/blob/main/GOVERNANCE.md#enhancement-proposals). We encourage you to explore either a formal proposal or implement this as a vendor specific extension and have them live & hosted on your own domain.

[0xtotaylor]

Thanks for the guidance, @yanheChen! I've updated this PR to follow the Enhancement Proposal process:

• Added rfcs/intent-trace.md with all required sections (Summary, Motivation, Detailed Design, Risks, Test Plan, Graduation Criteria)
• Updated the PR description to reflect Provisional status
• Implementation is included and ready to merge once the proposal reaches implementability.

Happy to address any feedback on the proposal. Requesting TC review when ready.