[codex] Fix NuGet README release links

[Widthdom]

Summary

• Generate the NuGet package README from docs/NUGET_README.md at pack time.
• Replace the README template placeholder with the package version tag, so NuGet pages link to matching release documentation instead of latest main.
• Point package release notes at the same version tag and add a changelog fragment.

Validation

• dotnet pack src/CodeIndex/CodeIndex.csproj --configuration Release --no-restore --output /private/tmp/cdidx-pack-check
• unzip -p /private/tmp/cdidx-pack-check/cdidx.1.24.5.nupkg NUGET_README.md confirmed documentation links use v1.24.5.
• unzip -p /private/tmp/cdidx-pack-check/cdidx.1.24.5.nupkg cdidx.nuspec confirmed release notes use v1.24.5.
• dotnet run --project tools/CodeIndex.Changelog -- check

Documentation and changelog

• Added changelog.d/unreleased/+nuget-readme-tag-links.fixed.md.
• Updated docs/NUGET_README.md to use the release-ref template placeholder.

Follow-up candidates

• None.