docs: update multi-backend plan - Phase 1 merged into /pre/v2.0

Author: claude

docs/design/multi-backend-plan.md

@@ -51,7 +51,7 @@ This document outlines the implementation plan for adding PostgreSQL support alo
 
 ## Phase 1: Core Type System Refactoring ✅ COMPLETE
 
-**Branch:** `feature/remove-unsigned-core-types`
+**Status:** Merged into `/pre/v2.0`
 
 ### 1.1 Unsigned Types Removed from Core Types