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/message_gateway/runner.go

@@ -0,0 +1,53 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import (
+	"context"
+	"sync"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/logger"
+)
+
+// Runner manages lifecycle for long-lived channel adapters (WebSocket, long-polling, etc.).
+type Runner struct {
+	mu      sync.Mutex
+	running bool
+	cancel  context.CancelFunc
+}
+
+// GlobalRunner is the default global runner instance.
+var GlobalRunner = &Runner{}
+
+// Start starts all background long-lived channel runners.
+func Start(ctx context.Context) error {
+	GlobalRunner.mu.Lock()
+	defer GlobalRunner.mu.Unlock()
+
+	if GlobalRunner.running {
+		return nil
+	}
+
+	runCtx, cancel := context.WithCancel(ctx)
+	GlobalRunner.cancel = cancel
+	GlobalRunner.running = true
+
+	logger.InfoF(runCtx, "[MessageGateway] Starting bot channel runners...")
+	return nil
+}
+
+// Stop stops the channel runner.
+func Stop() {
+	GlobalRunner.mu.Lock()
+	defer GlobalRunner.mu.Unlock()
+
+	if !GlobalRunner.running {
+		return
+	}
+
+	if GlobalRunner.cancel != nil {
+		GlobalRunner.cancel()
+	}
+	GlobalRunner.running = false
+}