build: improve the packaging metadata and add PR workflow

[behnazh-w]

This PR improves the build, docs, and testing as follows:

• Add PyPI metadata, including keywords, classifiers, and project URLs.
• Document Soufflé as an external dependency required for inference-based commands.
• Improve the missing-Soufflé error message and add a test for it.
• Add a PR GitHub Actions workflow to run tests.
• Add .gitignore rules for Python, Poetry, build, coverage, and usual generated analysis artifacts.

Closes #17