Convert to Markdown, rephrase, add short version

[iakov]

• Format Conversion: Converted the original LaTeX-based checklist to two Markdown files. Maintained all original guidance and principles from the LaTeX version
• repo-checklist-short.md - New concise version (10 items) for quick reference in PRs and code reviews, references full
• Rephrased sections for clarity in both versions (e.g., license and tests statements, 10th point in short version)
• SPDX License Headers: Added proper SPDX license headers (CC-BY-4.0) to both Markdown files for clear copyright attribution