Publish to Maven Central

[nficano]

Maven publishing is wired in lib/build.gradle.kts but no artifacts on Central yet. Once features are complete:

1. Configure OSSRH credentials in repo secrets
2. Add GitHub Actions publish workflow
3. Sign with GPG (require for Central)
4. Tag v1.1.0, run publish
5. Verify on https://search.maven.org/artifact/dev.arcp/arcp

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)