docs: add work log contributing guide

[vku2018]

Summary

• add a root CONTRIBUTING.md explaining the public Qortium Work Log contribution flow
• document issue/PR expectations, maintainer assignment/merge authority, and common labels/statuses
• link the guide from the README editing section

Validation

• npm ci
• npm run check
• npm run build
• README/CONTRIBUTING Markdown link check
• git diff --check

Notes

• npm ci reports 4 existing moderate npm audit advisories.
• npm run check passes with existing Astro/TypeScript hints about document.execCommand and window.qdnRequest.

Refs #8