Closes issue #393

[ferdinandre]

Added cover letter scoring in itself + against a given job describtion .txt file

Adds cover letter scoring as a standalone entry point, reusing the existing
--role system.
Usage
python score_cover_letter.py path/to/cover_letter.pdf --role software_engineering_intern

How it works

• Extracts the letter text with PyMuPDF get_text()
• Scores against four categories: job_alignment (40), motivation_fit (20),
  communication (20), specificity (20) plus bonus/deductions/strengths/
  improvements, matching the resume evaluation shape. Communication explicitly
  covers coherence, grammar, and professional phrasing.
• Prompts live per-role as cover_letter_criteria.jinja /
  cover_letter_system_message.jinja, same convention as the resume prompts.
• Optional roles/<role>/job_description.txt, if present the letter is scored
  against it; if absent, it's judged on its own against the role's position_title.

Files added

• score_cover_letter.py, cover_letter.py,
• roles/software_engineering_intern/cover_letter_criteria.jinja
• roles/software_engineering_intern/cover_letter_system_message.jinja