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>

apps/api/pkg/response/response.go

@@ -0,0 +1,25 @@
+// Package response provides the unified API envelope {code,message,data}.
+package response
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+)
+
+// Body is the standard JSON response shape for all HTTP APIs.
+type Body struct {
+	Code    int         `json:"code"`
+	Message string      `json:"message"`
+	Data    interface{} `json:"data,omitempty"`
+}
+
+// OK writes a success response with optional data.
+func OK(c *gin.Context, data interface{}) {
+	c.JSON(http.StatusOK, Body{Code: 0, Message: "success", Data: data})
+}
+
+// Fail writes a business error with HTTP status and app code.
+func Fail(c *gin.Context, httpStatus int, code int, message string) {
+	c.JSON(httpStatus, Body{Code: code, Message: message})
+}