v0.4.6 fix: cover letter is a real letter, not three paragraphs under a giant H1 (#64)

[earino]

Summary

Closes #64. Reshapes the cover-letter sub-agent prompt and the PDF renderer in lockstep so the generated cover-letter.md and cover-letter.pdf include every element a real business letter has: applicant header (name + contact line), today's date, company name, **Re:** {Position} subject line, plain-text greeting, three body paragraphs, Sincerely,, and the candidate's printed name with a 36pt signature gap.

Reported by @guiqvlaixi2164-max. The prior version explicitly told the LLM to omit those elements ("the student will add those themselves if needed") — so this was a deliberate-but-wrong design choice we're flipping.

Prompt (scripts/prompts.py):

• COVER_LETTER_PROMPT now requires 5 vars: contact_info, today_date, tailored_resume, jd_text, company_research. Output structure is locked into a 10-block schema in canonical order.
• Schema literals {Company} and {Position Title} pass through for the LLM to fill from the JD.

Orchestration (scripts/orchestration.py):

• build-prompt --kind cover-letter now reads .resumasher/config.json for the contact line (same builder used by tailor) and pre-formats today's date as Month D, YYYY server-side. The LLM cannot drift on either.
• New --today flag for test-time pinning.

Renderer (scripts/render_pdf.py):

• render_cover_letter redesigned. H1 = applicant name in resume's Name style; cover letter now visually matches the resume header so the packet reads as one designed pair.
• Three new styles: LetterBody (justified, 12pt spaceAfter), LetterDate (right-aligned per business-letter convention), LetterMeta (tighter for the company/Re: block).
• Bold **Re:** prefix renders via the existing inline-markdown pass. 36pt Spacer after Sincerely, creates real signature room. 18pt breather between header and date.
• Date detected by regex (full-block match only) and right-aligned.
• The previous # Dear {Company} Hiring Team, H1-as-greeting convention is retired — once a header exists above it, the giant H1 reads as broken.

Side effect on #63 (cover letter has a ragged right edge): LetterBody is TA_JUSTIFY, so the cover-letter PDF body is now justified. The resume summary paragraph still has the same issue — that's #63 proper, untouched here.

Consciously deferred:

• Street addresses (applicant or company): resumasher doesn't capture them, modern letters often skip them, and a student can add their own by editing the markdown.
• Inline handwritten-signature image: meaningful separate feature; will track as a follow-up issue.

Test Coverage

SAMPLE_COVER_MD rewritten to match the new structure. test_render_cover_letter_roundtrip now asserts every #64 element in the rendered PDF (name, contact, date, company, Re:, greeting, body content, Sincerely). test_render_bold_in_cover_letter updated to use the new shape and additionally verifies the bold **Re:** prefix renders correctly. test_cli_build_prompt_cover_letter exercises the new contact_info + today_date wiring through the orchestration CLI.

Test plan

• All 459 tests pass (sandbox, ~4min)
• Sample PDF rendered with realistic content; pdfminer round-trip confirms all structural elements present, in order; pypdf confirms 1 page
• Bold **Re:** markers convert to bold formatting (no literal ** in extracted text)
• Justified body paragraphs (no ragged right edge in cover letter)

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