Add release workflow (auto-version + PyPI publish)

[ldgaetano]

Create .github/workflows/release.yml to auto-release on push to main.

Jobs:

1. release — runs PSR GitHub Action to compute next version, stamp pyproject.toml, generate changelog, commit + tag, push, create GitHub release
2. build — depends on release. Checks out tagged commit, runs pixi install then pixi run build, uploads dist artifacts
3. pypi-publish — depends on build. Downloads artifacts and publishes to PyPI via pypa/gh-action-pypi-publish@release/v1 using OIDC trusted publishing

Part of the CI/CD setup plan.