refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/downstream/plugins/custom_example/plugin.go

@@ -0,0 +1,50 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package custom_example demonstrates how to build a downstream Cordis plugin.
+// Copy this directory to create your own plugin.
+package custom_example
+
+import (
+	"net/http"
+
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/core/contracts"
+	"github.com/gin-gonic/gin"
+)
+
+// Plugin implements core.Plugin for the custom_example downstream plugin.
+type Plugin struct{}
+
+// New creates a new custom_example plugin.
+func New() *Plugin {
+	return &Plugin{}
+}
+
+// Name returns the unique identifier for this plugin.
+func (p *Plugin) Name() string {
+	return "custom_example"
+}
+
+// Apply registers routes and services into the Cordis micro-kernel Context.
+func (p *Plugin) Apply(ctx *core.Context) error {
+	// Resolve platform services via IoC container (no direct imports of domain plugins).
+	var authSvc contracts.AuthService
+	if err := core.Using[contracts.AuthService](ctx, func(svc contracts.AuthService) { authSvc = svc }); err != nil {
+		return err
+	}
+	_ = authSvc
+
+	// Register routes using the auth middleware obtained through the contract.
+	g := ctx.Router().Group("/api/v1/custom", authSvc.RequireAuthMiddleware().(gin.HandlerFunc))
+	g.GET("/hello", func(c *gin.Context) {
+		user, err := authSvc.GetCurrentUser(c.Request.Context())
+		if err != nil {
+			c.JSON(http.StatusUnauthorized, gin.H{"error": "unauthorized"})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"message": "Hello " + user.Username})
+	})
+
+	return nil
+}