Add Packrift UCP shipping-supplies starter kit

[Packrift]

PR: Add Packrift UCP shipping-supplies starter kit

Summary

Adds Packrift to the Implementations section as a UCP-native packaging merchant and starter kit for shipping-supplies storefronts.

Packrift is relevant to the list because it exposes:

• Native Shopify UCP catalog/cart/checkout support.
• A public source-aware MCP endpoint for agent and storefront builders.
• A copy-ready shipping-supplies starter kit, starter SKUs, source slugs, shelf/import assets, and builder integration pack for fulfillment, warehouse, moving, and packing-station storefronts.

Fastest builder link:

https://mcp.packrift.com/ai/packrift-ucp-shipping-supplies-starter-kit.html

Integration pack:

https://mcp.packrift.com/ai/packrift-ucp-builder-integration-pack.html

Why

Most UCP examples are broad retail shopping demos. Packrift is a useful vertical implementation because packaging intent often needs SKU-level precision, dimensions, live inventory, pricing, and buyer-confirmed cart handoff.

Validation

The entry follows the existing one-resource-per-line README format.

[Packrift]

Closing this as a duplicate of #26, which is the canonical Packrift UCP shipping-supplies starter kit PR for review.