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>

package.json

@@ -42,6 +42,7 @@
     "vitest": "^4.0.18"
   },
   "dependencies": {
+    "@multica/sdk": "workspace:*",
     "@mariozechner/pi-agent-core": "^0.50.3",
     "@mariozechner/pi-ai": "^0.50.3",
     "@mariozechner/pi-coding-agent": "^0.50.3",