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/cmd/api.go

@@ -0,0 +1,23 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cmd
+
+import (
+	"log"
+
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/spf13/cobra"
+)
+
+var apiCmd = &cobra.Command{
+	Use:   "api",
+	Short: "wavelet API",
+	Run: func(_ *cobra.Command, _ []string) {
+		app := newWaveletApp(core.ProfileAPI)
+		if err := app.Run(); err != nil {
+			log.Fatalf("[API] run failed: %v\n", err)
+		}
+	},
+}