fix: correct NuGet repository URLs

[ddeml]

Summary

• correct PackageProjectUrl and RepositoryUrl to point to https://github.com/drmathias/robots
• ensure future NuGet packages link to the actual project and source repository

Root cause

The package metadata still referenced the nonexistent drmathias/Robots.Txt.Parser repository.

Impact

Future releases will display working project and source links on NuGet.org. Already-published package metadata remains unchanged until a new version is released.

Validation

• dotnet build .\Robots.Txt.Parser.slnx -c Release passed with no warnings or errors
• dotnet pack .\src\Robots.Txt.Parser\Robots.Txt.Parser.csproj -c Release --no-build passed
• inspected the generated .nuspec; both projectUrl and repository url contain https://github.com/drmathias/robots
• full test run: 149 passed; 6 unrelated existing tests failed in size-boundary and locale-sensitive parsing scenarios

Closes #1