docs: add gift extension draft

[seungy0]

Description

Adds an exploratory Gift Extension note for recipient-based gifting flows.

In recipient-based gift shopping, the buyer purchases a product for another
recipient. The protocol difference from ordinary shopping is that recipient
selection can happen before product selection, after product selection, or
during checkout.

The draft focuses on a privacy-preserving shape:

• business-hosted recipient selection,
• reuse of continue_url for product-before-recipient handoff,
• opaque recipient selection tokens,
• carrying selected recipient state into checkout,
• no raw friend list, address book, social graph data, or stable recipient
  identifier exposure to platforms or agents,
• checkout-time validation before payment completion.

This is intentionally not a complete schema PR. It is intended to collect
protocol feedback before schema work:

1. Is dev.ucp.shopping.gift the right common capability boundary for gift
   shopping flows?
2. Should v1 remain checkout-only?
3. Is Checkout continue_url sufficient for product-before-recipient flows,
   and does UCP need a more explicit initiation or return contract for
   recipient-before-product flows?

No JSON schema is added in this PR. The goal is to validate the capability
boundary and flow model before proposing a normative schema.

Refs #449

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentation regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Checklist

• I have followed the Contributing Guide.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

The schema/model checklist items are not applicable for this docs-only draft.