fix(pdf): handle partially numbered lists in PDF conversion (#68)

[pallaprolus]

Fixes #68

This PR addresses the issue where partially numbered lists (common in MasterFormat documents, e.g., .1, .2) are extracted as plain text lines indistinguishable from regular paragraphs.

Changes:

• Adds a lightweight regex post-processing step in _pdf_converter.py to identify lines starting with .Number and convert them into Markdown lists (- .Number).
• This keeps the solution dependency-free and lightweight as requested by maintainers.

Verification:

• Verified that lines like .1 Item are now converted to - .1 Item.
• Ran standard tests to ensure no regressions.