feat: add build-test-dotnet workflow for .NET/NuGet validation (#636)

Add agentic workflow to clone and build .NET test projects through
the AWF firewall, validating NuGet package restore and dotnet build
work correctly with proxy configuration.

- Create build-test-dotnet.md workflow (dotnet domain preset)
- Compile and postprocess lock file
- Add to postprocess-smoke-workflows.ts
- Test repo: Mossaka/gh-aw-firewall-test-dotnet (hello-world + json-parse)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Mossaka