fix: use dedicated NuGet package README

[Cynrath]

Adds README.nuget.md as the NuGet package README, keeps README.md as the GitHub README, updates package metadata validation, and documents the README ownership boundary for future agents.

Validation:

• scripts/check-package-metadata.ps1 -FailOnIssues passed
• dotnet build Release passed
• dotnet test Release passed: 428/428
• dotnet pack Release passed
• nupkg contains README.nuget.md

Notes:

• No NuGet publish.
• No tag or GitHub Release mutation.
• Current CLI/package version intentionally remains 0.2.0-alpha.3.