Add GitHub Actions link-check workflow using lychee

[curiouscoder-cmd]

Summary of your changes

Description

• Adds a CI workflow at .github/workflows/link-check.yml to validate links in README.md using lycheeverse/lychee-action@v1.
• Excludes domains/URLs listed in white_listed_sites.txt via --exclude-file.
• Uploads a lychee report artifact for easier review.
• Aims to prevent broken links and improve repository quality.

Checklist

• [ ✅ ] My change follows the Contributing Guidelines
• I have added only one new link to the list. (N/A — CI/config change, no links added)
• I have checked that the link that I added does NOT exist in the project already. (N/A)
• I have sorted the link alphabetically under the related section. (N/A)