Add GitHub Actions summary renderer

[johnmikel]

Summary

• Add a GitHub Actions Markdown summary renderer for contract run results.
• Add metricspec run --github-summary to append output to $GITHUB_STEP_SUMMARY.
• Document the flag in the GitHub Actions CI example.

Closes #3.

Verification

• uv run ruff check .
• uv run mypy src
• uv run pytest
• uv run python -m build
• uv run twine check dist/*