feat: implement pdf-to-md-swift converter

[PsychQuantClaw]

Summary

Restore and finish pdf-to-md-swift as a proper Layer 3 converter in macdoc.

The current main branch only carries the package manifest / README / CLI wiring from the earlier merge, but the tracked converter source and tests are missing. This follow-up issue closes that gap by landing the actual implementation.

Conversion Requirements

• Input: .pdf
• Output: Markdown stream / .md
• Architecture: direct PDF → Markdown path, avoiding hub loss through LaTeX
• Protocol: implement DocumentConverter with StreamingOutput

Layer 1 / Core Dependencies

• PDFKit for native macOS PDF parsing
• common-converter-swift (DocumentConverter, StreamingOutput, ConversionOptions)

Implementation Notes

• Extract PDF content page-by-page
• Heuristically map headings, paragraphs, ordered lists, unordered lists
• Preserve page boundaries with Markdown thematic breaks
• Support frontmatter + hard line break options already exposed in CLI
• Keep logic in packages/pdf-to-md-swift/ as an independent Swift package

Test Strategy

• Package-level unit tests for headings / paragraphs / list detection
• Page-break handling across multi-page PDFs
• Hyphenated line join behavior
• Frontmatter and hard-break options
• swift test inside packages/pdf-to-md-swift

[PsychQuantClaw]

Opened follow-up PR: #51