Publish to PyPI with release automation

[rosshd]

Objective

Make `pipx install openlearn` work.
Today install requires git clone plus `pip install -e .` and the package is not on PyPI.

Scope

• Publish the package to PyPI under the `openlearn` name.
• Add a tag-triggered GitHub Actions release workflow: build sdist/wheel, publish via trusted publishing, attach artifacts to the GitHub release.
• Single-source the version (pyproject is currently 0.6.0; CLI strings have drifted before).
• Bump the Development Status classifier when 1.0 ships.

Verification

• `pipx install openlearn` on a clean machine gives a working `openlearn` command.
• Tagging `vX.Y.Z` produces a PyPI release with no manual steps.

[rosshd]

Board hygiene status: the tag-triggered release workflow is present on origin/main, but no published v0.6.0 GitHub release was found. Leaving this open for the final tag, GitHub release, PyPI publish, and clean pipx-install verification.