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/banner_test.go

@@ -0,0 +1,47 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cmd
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/buildinfo"
+	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+)
+
+func TestFormatStartupBanner(t *testing.T) {
+	previousVersion := buildinfo.Version
+	previousBuildTime := buildinfo.BuildTime
+	previousEnv := config.Config.App.Env
+	previousAddr := config.Config.App.Addr
+	t.Cleanup(func() {
+		buildinfo.Version = previousVersion
+		buildinfo.BuildTime = previousBuildTime
+		config.Config.App.Env = previousEnv
+		config.Config.App.Addr = previousAddr
+	})
+
+	buildinfo.Version = "v3.2.1"
+	buildinfo.BuildTime = "2026-07-13T08:00:00Z"
+	config.Config.App.Env = "production"
+	config.Config.App.Addr = ":3000"
+
+	banner := formatStartupBanner(startupState{
+		mode:           "API",
+		listensForHTTP: true,
+	})
+
+	for _, want := range []string{
+		"Wavelet v3.2.1",
+		"Environment: production",
+		"Build time:  2026-07-13T08:00:00Z",
+		"Listening:   http://:3000",
+		"Mode:        API",
+	} {
+		if !strings.Contains(banner, want) {
+			t.Errorf("banner missing %q:\n%s", want, banner)
+		}
+	}
+}