Add OpenSpec proposal for converting markdown cover letters to PDF format using Swiss business letter standards.
The proposal includes:
- Swiss letter formatting with scrlttr2 and Swiss Norm configuration
- Environment validation for Pandoc and LaTeX
- Comprehensive error handling and user guidance
- Support for Swiss German formatting and address window positioning
This addresses step 7 "Generate PDFs" in the project workflow.
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Co-Authored-By: Claude <noreply@anthropic.com>