feat(ci): add GitHub Actions CI for all 6 samples

4 jobs run in parallel:
- validate: matrix of 3 use-case samples (load config + run test.sh)
- validate-digital-twins: matrix of Stripe/Twilio/OpenAI (load + smoke)
- docker: docker compose up/down with endpoint verification
- mcp-sdk: MCP Python SDK integration test (15 assertions)

Installs mockd from latest release, validates configs load, runs
CRUD smoke tests, verifies prefix IDs and response envelopes.

Author: zach-snell

ai-agents/mcp-workflow/test_mcp_sdk.py

@@ -135,7 +135,7 @@ async def main():
             # 4. Verify with HTTP requests
             print("\n--- Verify CRUD via HTTP ---")
             # Use the port from status, not hardcoded
-            mock_port = 7280  # matches our running server
+            mock_port = 4280  # default
             for p in status.get("ports", []):
                 if p.get("component") == "Mock Engine":
                     mock_port = p["port"]