Infrastructure: Generic GTFS Diff Engine publish package

[cka-y]

Describe the problem

Once the standalone gtfs_diff package exists, there is no automated path for publishing it to PyPI. Without a release pipeline, distribution would rely on manual uploads, which is error-prone and blocks the package from being easily consumed by downstream projects (including the MobilityData-specific orchestrator).

Proposed solution

Set up automated PyPI publishing for the gtfs_diff repository via GitHub Actions. Releases trigger on tags matching v*, using pypa/gh-action-pypi-publish and a PYPI_API_TOKEN repository secret.

Acceptance criteria:

• GitHub Actions workflow triggers on version tag push (v*)
• Workflow uses pypa/gh-action-pypi-publish to publish the package
• PYPI_API_TOKEN is configured as a repository secret Use Trusted Publisher instead of the PIP TOKEN
• Package is installable via pip install <package-name> after a tagged release
• README.md install instructions reflect the published PyPI name
• Release process documented (how to cut a version tag)

[Alessandro100]

There is a way to accomplish this without the PYPI_API_TOKEN