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/errs.go

@@ -0,0 +1,26 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package message_gateway
+
+import "errors"
+
+var (
+	errCodeInvalid          = errors.New("invalid or expired pairing code")
+	errChannelMismatch      = errors.New("pairing code does not match channel")
+	errPlatformAlreadyBound = errors.New("this platform account is already bound")
+	errBindingNotFound      = errors.New("binding not found")
+	errBindingForbidden     = errors.New("cannot unbind another user's binding")
+	errChannelIDRequired    = errors.New("channel_id is required")
+	errChannelDisabled      = errors.New("channel is not enabled")
+)
+
+const (
+	errNameRequired          = "name is required"
+	errTypeInvalid           = "type must be telegram or qq"
+	errTelegramTokenRequired = "telegram bot secret is required"   //nolint:gosec // user-facing validation text
+	errQQCredentialsRequired = "qq app id and secret are required" //nolint:gosec // user-facing validation text
+	errChannelNotFound       = "channel not found"
+	errChannelProbeFailed    = "channel probe failed"
+	maskedSecret             = "********"
+)