chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/adr/0002-use-gin.md

@@ -0,0 +1,19 @@
+# ADR-0002 Use gin HTTP framework
+
+## Status
+
+Accepted
+
+## Decision
+
+Use `github.com/gin-gonic/gin` for HTTP routing and middleware.
+
+## Reason
+
+1. Widely known; good AI completion quality
+2. Enough features for REST MVP
+3. Low ceremony
+
+## Never
+
+Never switch to echo/fiber/chi “for fun” mid-feature. Framework change needs a new ADR.