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/package.json

@@ -7,8 +7,10 @@
   "exports": {
     ".": {
       "types": "./dist/index.d.ts",
-      "import": "./dist/index.js"
-    }
+      "import": "./dist/index.js",
+      "default": "./dist/index.js"
+    },
+    "./*": "./src/*.ts"
   },
   "scripts": {
     "build": "tsc",