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.go

@@ -0,0 +1,55 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package cmd provides CLI command entry points.
+//
+//nolint:unused
+package cmd
+
+import (
+	"fmt"
+	"log"
+	"runtime"
+	"strings"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/buildinfo"
+	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+)
+
+//nolint:unused // startup banner formatting utilities
+type startupState struct {
+	mode           string
+	listensForHTTP bool
+}
+
+func printStartupBanner(state startupState) {
+	log.Print(formatStartupBanner(state))
+}
+
+func formatStartupBanner(state startupState) string {
+	lines := []string{
+		"",
+		"__        __                _      _   ",
+		"\\ \\      / /_ ___   _____  | | ___| |_ ",
+		" \\ \\ /\\ / / _` \\ \\ / / _ \\ | |/ _ \\ __|",
+		"  \\ V  V / (_| |\\ V /  __/ | |  __/ |_ ",
+		"   \\_/\\_/ \\__,_| \\_/ \\___|_|\\___|\\__|",
+		fmt.Sprintf(" Wavelet %s", buildinfo.Version),
+		"",
+		fmt.Sprintf(" Environment: %s", config.Config.App.Env),
+		fmt.Sprintf(" Runtime:     %s/%s (%s)", runtime.GOOS, runtime.GOARCH, runtime.Version()),
+		fmt.Sprintf(" Build time:  %s", buildTime()),
+	}
+	if state.listensForHTTP {
+		lines = append(lines, fmt.Sprintf(" Listening:   http://%s", config.Config.App.Addr))
+	}
+	lines = append(lines, fmt.Sprintf(" Mode:        %s", state.mode), "")
+	return strings.Join(lines, "\n")
+}
+
+func buildTime() string {
+	if buildinfo.BuildTime == "" {
+		return "development build"
+	}
+	return buildinfo.BuildTime
+}