Improve README and virtualenv hygiene checks

[ahmetmizrap1232-sys]

Summary

• Ignore Markdown headings inside fenced code blocks when checking README sections.
• Report common local Python virtualenv folders (.venv/, venv/, env/) as junk unless they are ignored.
• Document existing CI-friendly exit codes and cover them with tests.

Closes #8

Validation

• python -m unittest discover

[omerbek]

Thanks for the contribution. This PR overlaps with changes that have now landed separately:

• Ignore README headings inside code blocks #9 fixed README heading detection inside fenced code blocks and closed Avoid treating Markdown code block comments as README headings #6.
• Report unignored Python virtualenv folders #11 added explicit handling for .venv/, venv/, and env/ folders and closed Consider reporting common Python virtual environment folders #7.

I tested this branch against current main, and it now has a conflict in repodx.py. Since the behavioral fixes are already merged and the remaining README/exit-code test additions are optional follow-up polish, I am closing this PR as superseded rather than resolving the conflict here.