Remove unused Slang compiler CLI

[botoddly]

Summary

• remove the unused GameKit.SdlangCompilerCli project and its System.CommandLine dependency
• remove standalone Slang distribution APIs and build targets that existed only for the CLI
• run compiler tests against the build-owned Slang installation without copying native tooling into test output
• update shader documentation for build-time-only compiler usage

GameKit.SdlangCompileLib and GameKit.SdlangCompileTask remain separate in this change. Their package-time structure will be evaluated as part of the NuGet publishing work.

Verification

• dotnet build GameKit.slnx -p:TreatWarningsAsErrors=true
• dotnet test GameKit.slnx --no-build -p:TreatWarningsAsErrors=true
• dotnet test tests/GameKit.SdlangBuildIntegration.Tests/GameKit.SdlangBuildIntegration.Tests.csproj --no-build --filter 'TestCategory=BuildIntegration' -p:TreatWarningsAsErrors=true