Set up PyPI publish workflow

[nficano]

pyproject.toml declares v1.1.0 but no GitHub Actions publish workflow exists. PyPI status unverified.

Steps:

1. Add .github/workflows/publish.yml triggered on v* tags
2. Use PyPI Trusted Publishing (OIDC) — configure on PyPI side
3. Build with uv build or hatch build (whichever matches project setup)
4. Tag v1.1.0 and verify package appears at https://pypi.org/project/arcp/

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