Add fail-on severity threshold and GitHub Actions example

[bharqav]

Description

I added a new fail on severity threshold feature to the CLI so that scans can automatically fail the CI pipeline if findings meet or exceed a specific severity level. I also included a GitHub Actions example configuration file so users can easily see how to integrate this tool into their own workflows. This makes it much easier for teams to enforce security rules during their build process.

Fixes #52

Type of change

✓ New feature (non breaking change which adds functionality)

How Has This Been Tested?

I ran the full test suite locally to verify the new threshold logic behaves correctly across different severity levels. I also ran the linter and type checker to make sure the code quality remains high. Here are the exact metrics and results:

✓ Pytest executed successfully with 280 total tests passing in 19.44 seconds.
✓ The newly added CLI tests for the fail on feature passed perfectly, specifically verifying the threshold logic:
✓ test scan fail on high fails with critical
✓ test scan fail on high fails with high
✓ test scan fail on high passes with medium
✓ test scan fail on invalid severity
✓ Ruff check ran across the entire project and reported all checks passed with zero remaining errors.
✓ Ruff format confirmed that the formatting and style guidelines are correctly applied.

✓ pytest tests/test_cli.py
✓ mypy crucible/ tests/ strict
✓ ruff check crucible/

Checklist:

✓ My code follows the style guidelines of this project
✓ I have performed a self review of my own code
✓ I have commented my code, particularly in hard to understand areas
✓ I have made corresponding changes to the documentation
✓ My changes generate no new warnings
✓ I have added tests that prove my fix is effective or that my feature works
✓ New and existing unit tests pass locally with my changes
✓ Any dependent changes have been merged and published in downstream modules