Add smoke tests for demos

## Summary

Add automated smoke tests to CI that verify demos actually start and the gateway API works, not just that Docker images build.

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## Proposed solution

Current CI:

- ✅ Docker image builds
- ✅ YAML/shell lint
- ❌ Demo starts successfully
- ❌ Gateway responds
- ❌ Nodes discovered
- ❌ API works

Proposed Tests:

- Smoke Test (Every PR and nightly)
