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/patterns/handler.md

@@ -0,0 +1,37 @@
+# Pattern: Handler
+
+## Responsibility
+
+HTTP only: bind JSON/query → validate → call service interface → map errors to envelope.
+
+## Shape
+
+```go
+type ReportService interface {
+    Decode(ctx context.Context, in DecodeInput) (*DecodeOutput, error)
+}
+
+type ReportHandler struct {
+    svc ReportService
+}
+
+func (h *ReportHandler) Decode(c *gin.Context) {
+    var req DecodeRequest
+    if err := c.ShouldBindJSON(&req); err != nil {
+        response.Fail(c, 400, 10000, "invalid request")
+        return
+    }
+    out, err := h.svc.Decode(c.Request.Context(), toInput(req))
+    if err != nil {
+        // map domain errors → codes
+        response.Fail(c, 400, 30001, err.Error())
+        return
+    }
+    response.OK(c, out)
+}
+```
+
+## Never
+
+- SQL here
+- Business unlock math here (belongs service)