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/examples/good-handler.go

@@ -0,0 +1,54 @@
+//go:build ignore
+
+// EXAMPLE — reference shape for AI. Not compiled into apps/api.
+package examples
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+type apiBody struct {
+	Code    int         `json:"code"`
+	Message string      `json:"message"`
+	Data    interface{} `json:"data,omitempty"`
+}
+
+type DecodeInput struct {
+	Year  int
+	Month int
+	Day   int
+}
+
+type DecodeOutput struct {
+	Summary string `json:"summary"`
+}
+
+type DecodeService interface {
+	Decode(ctx context.Context, in DecodeInput) (*DecodeOutput, error)
+}
+
+type DecodeHandler struct {
+	Svc DecodeService
+}
+
+// Decode handles POST /api/v1/reports/decode
+func (h *DecodeHandler) Decode(c *gin.Context) {
+	var req struct {
+		Year  int `json:"year" binding:"required"`
+		Month int `json:"month" binding:"required"`
+		Day   int `json:"day" binding:"required"`
+	}
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, apiBody{Code: 10000, Message: "invalid request"})
+		return
+	}
+	out, err := h.Svc.Decode(c.Request.Context(), DecodeInput(req))
+	if err != nil {
+		c.JSON(http.StatusBadRequest, apiBody{Code: 30001, Message: err.Error()})
+		return
+	}
+	c.JSON(http.StatusOK, apiBody{Code: 0, Message: "success", Data: out})
+}