Add structured HTML diff report outputs

[ryan-reid]

Summary

• emit structured JSON DTOs for item and Excel diff reports
• keep Markdown as a renderer rather than the canonical data layer
• add browser-friendly HTML report pages generated from the same DTO data
• publish generated report pages into the wiki under /reports/ so GitHub Pages can serve them
• add shared generated report CSS and ignore local generated report HTML/JSON artifacts outside the published wiki
• add unit coverage for HTML report generation and wiki report publishing

Verification

• python -m unittest discover -s tests
• python scripts/generate_reports.py
• browser-checked /reports/ and /reports/excel/retail-bk/gems.html over localhost