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/plugins/domain/admin/restart_unix.go

@@ -0,0 +1,50 @@
+//go:build !windows
+
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package admin
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+)
+
+const installedBinaryMode = 0o755
+
+func replaceAndRestart(executable, stagedBinary string) error {
+	ctx := context.Background()
+	logger.InfoF(ctx, "[Updater] Swapping executable: %s -> %s", executable, stagedBinary)
+	backup := executable + ".old"
+
+	if err := os.Remove(backup); err != nil && !os.IsNotExist(err) {
+		return fmt.Errorf("删除旧备份失败: %w", err)
+	}
+
+	if err := os.Rename(executable, backup); err != nil {
+		return fmt.Errorf("备份当前程序失败: %w", err)
+	}
+
+	if err := os.Rename(stagedBinary, executable); err != nil {
+		_ = os.Rename(backup, executable)
+		return fmt.Errorf("替换当前程序失败: %w", err)
+	}
+
+	if err := os.Chmod(executable, installedBinaryMode); err != nil {
+		_ = os.Remove(executable)
+		_ = os.Rename(backup, executable)
+		return fmt.Errorf("设置程序执行权限失败: %w", err)
+	}
+
+	stagingDir := filepath.Dir(stagedBinary)
+	_ = os.RemoveAll(stagingDir)
+
+	logger.InfoF(ctx, "[Updater] Executing syscall.Exec to restart service: %s %v", executable, os.Args)
+	//nolint:gosec // restart process via exec with same binary and args
+	return syscall.Exec(executable, os.Args, os.Environ())
+}