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/channel.go

@@ -0,0 +1,21 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import "context"
+
+// Handler processes one inbound message.
+type Handler func(ctx context.Context, msg InboundMessage) error
+
+// Factory constructs a Channel from decrypted config.
+type Factory func(cfg ChannelConfig, onInbound Handler) (Channel, error)
+
+// Channel is one connected messaging adapter.
+type Channel interface {
+	Type() string
+	Connect(ctx context.Context) error
+	Disconnect(ctx context.Context) error
+	Send(ctx context.Context, to Recipient, msg OutboundMessage) error
+	Capabilities() Capability
+}