Single-source the version and add a PyPI release workflow

[adaumsilva]

Description

The version is declared twice — pyproject.toml (version = "0.1.0") and ragframework/__init__.py (__version__ = "0.1.0") — and will drift. There is also no release automation; the README says "Once published to PyPI, installation will simplify to pip install ragframework", so publishing is the goal.

Acceptance criteria

• ragframework/__init__.py derives __version__ from importlib.metadata.version("ragframework") (with a PackageNotFoundError fallback of "0.0.0+unknown" for source checkouts that aren't installed); pyproject.toml remains the single source of truth
• Test that ragframework.__version__ matches the installed metadata
• .github/workflows/release.yml triggered on v* tags: builds sdist + wheel with python -m build, runs twine check, publishes with PyPI Trusted Publishing (pypa/gh-action-pypi-publish, permissions: id-token: write, no long-lived secrets), and creates a GitHub Release with notes extracted from the matching CHANGELOG.md section
• CONTRIBUTING.md / a short RELEASING.md: bump version in pyproject.toml → move [Unreleased] to a dated section → tag → push
• Maintainer action required (not in the PR): register the project on PyPI and configure the trusted publisher for this repo/workflow
• README install instructions updated once the first release is out

Files to touch

• ragframework/__init__.py
• .github/workflows/release.yml — new
• RELEASING.md — new (or a section in CONTRIBUTING.md)
• tests/test_version.py — new

Resources

• importlib.metadata
• Publishing to PyPI with Trusted Publishing
• pypa/gh-action-pypi-publish

Estimated effort: Medium (half day)