Mirrors the website repo's setup: a source-based scan of sidecar/src that
reports to the self-hosted SonarQube server after merge, never gating PRs.
Uses the existing SonarQube project key Runic-Gateway-link (the server
rejects re-creating a case-variant key). Supersedes #13.
Co-Authored-By: Claude <noreply@anthropic.com>
