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/main.go

@@ -0,0 +1,22 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package main 是 Wavelet 平台的程序入口
+package main
+
+import "github.com/Rain-kl/Wavelet/backend/cmd"
+
+// @title Wavelet API
+// @version 1.0.0
+// @description Wavelet 平台后端 API，提供用户认证、系统配置、任务调度等通用功能。
+// @contact.name Wavelet
+// @contact.url https://github.com/Rain-kl/Wavelet
+// @license.name Apache 2.0
+// @license.url http://www.apache.org/licenses/LICENSE-2.0.html
+// @BasePath /
+// @securityDefinitions.apikey SessionCookie
+// @in cookie
+// @name session
+func main() {
+	cmd.Execute()
+}