ci: add PyPI publishing workflows

[yeiichi]

Summary

Adds GitHub Actions CI and PyPI publishing workflows.

Details

• Adds a CI workflow for pull requests and pushes to main.
• Runs tests on Python 3.10, 3.11, and 3.12.
• Adds a package build job that builds the wheel/sdist and checks them with Twine.
• Adds a tag-triggered PyPI publish workflow for v* tags.
• Uses PyPI Trusted Publishing via GitHub OIDC, so no PyPI API token secret is needed.
• Documents the required PyPI Trusted Publisher setup and tag-based release flow.

Manual setup required

Before the first automated PyPI publish, configure the PyPI Trusted Publisher:

• Owner: yeiichi
• Repository: csvsmith
• Workflow name: pypi.yml
• Environment name: pypi

Testing

Validated locally with:

uv run --with pytest pytest
uv run --with build python -m build --outdir /tmp/csvsmith-dist
uv run --with twine twine check /tmp/csvsmith-dist/*