Convert README.md links to absolute URLs for NuGet package compatibility

[Copilot]

• Convert relative links in README.md to absolute URLs
  • Update link to CONTRIBUTING.md (2 occurrences)
  • Update link to CODE_OF_CONDUCT.md
  • Update link to SECURITY.md
  • Update link to AGENTS.md
  • Update link to LICENSE
• Refactor to use Markdown link references for cleaner code
• Remove QUALITY_IMPROVEMENTS_SUMMARY.md file
• Verify all changes and test that links still work in the repository

Original prompt

please modify the links in the README.md file to be absolute URLs so the links work when the file is packaged in the nuget package and viewed on nuget.org.

Additionally please remove the QUALITY_IMPROVEMENTS_SUMMARY.md file as changes are automatically included in the release notes.

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[Malcolmnixon]

Consider using link-references

[Copilot]

Consider using link-references

Updated to use Markdown link references. All absolute URLs are now defined at the bottom of the README.md for easier maintenance. (62cd13d)