Add CI workflow to run tests across contracts and integrations

[Thaleszh]

Is your feature request related to a problem? Please describe.
There’s no automated CI testing pipeline defined, risking broken master.

Describe the solution you'd like
Add GitHub Actions YAML under .github/workflows/ci.yml that executes:

• cargo test in contracts/ folders
• cargo test in integration-tests/
• Lint checks via cargo clippy