refactor(sdk): unify gateway-sdk into @multica/sdk package

Sync latest code from src/shared/gateway-sdk/ into packages/sdk/,
update all backend imports to use @multica/sdk, and remove the
duplicate src/shared/gateway-sdk/ directory.

- Translate Chinese comments to English in SDK source
- Fix package.json exports with default condition
- Add @multica/sdk as workspace dependency for backend
- Update imports in gateway, test-client, and hub

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gateway/events.gateway.ts

@@ -21,7 +21,7 @@ import {
   type PongResponse,
   type DeviceInfo,
   type DeviceType,
-} from "../shared/gateway-sdk/index.js";
+} from "@multica/sdk";
 
 @Injectable()
 @WebSocketGateway({

src/gateway/test-client.ts

@@ -4,7 +4,7 @@ import {
   HelloResponseAction,
   type HelloPayload,
   type HelloResponsePayload,
-} from "../shared/gateway-sdk/index.js";
+} from "@multica/sdk";
 
 // 模拟一个 Client
 const client = new GatewayClient({