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/registry.go

@@ -0,0 +1,26 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import "sync"
+
+var (
+	factoriesMu sync.RWMutex
+	factories   = map[string]Factory{}
+)
+
+// Register stores a channel factory under typ.
+func Register(typ string, fn Factory) {
+	factoriesMu.Lock()
+	defer factoriesMu.Unlock()
+	factories[typ] = fn
+}
+
+// Lookup returns a previously registered factory.
+func Lookup(typ string) (Factory, bool) {
+	factoriesMu.RLock()
+	defer factoriesMu.RUnlock()
+	fn, ok := factories[typ]
+	return fn, ok
+}