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)

.agents/skills/new-async-task/SKILL.md

@@ -26,7 +26,7 @@ description: "Wavelet 项目专用：新增或修改基于 Cordis 插件的 Asyn
 
 ### 步骤 1：定义任务 Payload 结构与类型常量
 
-在插件内（如 `plugins/domain/order/tasks.go`）：
+在插件内（如 `backend/plugins/domain/order/tasks.go`）：
 
 ```go
 package order

.agents/skills/new-async-task/references/CODE-EXAMPLES.md

@@ -6,7 +6,7 @@
 
 ## 1. 任务定义与 Handler 编写
 
-在对应的业务插件中（如 `plugins/domain/user/tasks.go`）：
+在对应的业务插件中（如 `backend/plugins/domain/user/tasks.go`）：
 
 ```go
 package user
@@ -61,8 +61,8 @@ package user
 import (
 	"time"
 
-	"github.com/Rain-kl/Wavelet/core"
-	"github.com/Rain-kl/Wavelet/core/extpoints"
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/core/extpoints"
 )
 
 func (p *Plugin) Apply(ctx *core.Context) error {