docs: remove cart-specific entry, mark Universal-Commerce-Protocol#17 and Universal-Commerce-Protocol#18 as not implemented

Cart capability was not part of the 2026-01-23 spec (introduced 2026-01-26),
so cart-to-checkout (Universal-Commerce-Protocol#13) is new capability work, not a migration concern.

Eligibility claims (Universal-Commerce-Protocol#17) and signals (Universal-Commerce-Protocol#18) are not yet implemented in Shopify.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: alexpark20

ucp-spec-diff/mcp-implementation-guide.md

@@ -23,10 +23,9 @@
 | 7 | Profile `Cache-Control: public, max-age ≥ 60` | draft | `2026-01-23, draft` | ✅ | [#156](https://github.com/Universal-Commerce-Protocol/ucp/pull/156) |
 | 10 | `adjustment.amount` minimum: 0 enforcement | draft | `—` | ✅ | [#246](https://github.com/Universal-Commerce-Protocol/ucp/pull/246) |
 | 12 | New error codes (`out_of_stock`, `address_undeliverable`) | draft | `2026-01-23, draft` | ✅ | [#147](https://github.com/Universal-Commerce-Protocol/ucp/pull/147) |
-| 13 | Cart-to-checkout: discount codes forwarded explicitly | draft | `2026-01-23, draft` | ✅ | [#246](https://github.com/Universal-Commerce-Protocol/ucp/pull/246) |
 | 16 | Signing headers (`meta.signature`, `meta.idempotency-key`, `meta.ucp-agent`) | draft | `2026-01-23, draft` | ✅ | [#156](https://github.com/Universal-Commerce-Protocol/ucp/pull/156) |
-| 17 | Eligibility claims & `invalid_eligibility` blocking error | draft | `2026-01-23, draft` | ✅ | [#250](https://github.com/Universal-Commerce-Protocol/ucp/pull/250) |
-| 18 | `signals` field on cart/checkout | draft | `2026-01-23, draft` | ✅ | [#203](https://github.com/Universal-Commerce-Protocol/ucp/pull/203) |
+| 17 | Eligibility claims & `invalid_eligibility` blocking error | draft | `2026-01-23, draft` | ❌ | [#250](https://github.com/Universal-Commerce-Protocol/ucp/pull/250) |
+| 18 | `signals` field on cart/checkout | draft | `2026-01-23, draft` | ❌ | [#203](https://github.com/Universal-Commerce-Protocol/ucp/pull/203) |
 | 19 | Fulfillment method `id`/`type` optional on update | draft | `2026-01-23, draft` | ✅ | [#143](https://github.com/Universal-Commerce-Protocol/ucp/pull/143), [#196](https://github.com/Universal-Commerce-Protocol/ucp/pull/196) |
 | 20 | `intent` field in context | draft | `2026-01-23, draft` | ✅ | [#95](https://github.com/Universal-Commerce-Protocol/ucp/pull/95) |