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>

packages/sdk/src/actions/hello.ts

@@ -1,14 +1,14 @@
-/** Hello Action - 测试用的问候消息 */
+/** Hello Action - test greeting message */
 
 export const HelloAction = "hello" as const;
 export const HelloResponseAction = "hello_response" as const;
 
-/** Hello 请求 payload */
+/** Hello request payload */
 export interface HelloPayload {
   greeting: string;
 }
 
-/** Hello 响应 payload */
+/** Hello response payload */
 export interface HelloResponsePayload {
   reply: string;
 }