Add minimal MSBuild-based C# Hello World sample with CI build workflow

[Copilot]

Adds a minimal C# console sample that can be built via MSBuild and wires CI to build it on every PR/push.
This introduces a small, explicit project layout for a baseline MSBuild-backed .NET repo.

• Project scaffold

  • Added HelloWorld.sln (classic .sln format) and src/HelloWorld/HelloWorld.csproj
  • Added Program.cs with a minimal Hello World entrypoint
  • Target framework set to net8.0 for stable CI/runtime compatibility

• GitHub Actions (MSBuild)

  • Added .github/workflows/build.yml
  • Workflow runs on push to main and pull_request
  • Uses actions/setup-dotnet + microsoft/setup-msbuild
  • Builds solution via MSBuild (Restore,Build) in Release

• Repo hygiene / docs

  • Added .gitignore entries for bin/ and obj/
  • Updated README.md with MSBuild build command and local run command

- name: Restore and build solution with MSBuild
  run: msbuild HelloWorld.sln /t:Restore,Build /p:Configuration=Release

Original prompt

Create a sample C# "hello world" console application to be build using MSBuild, with a GitHub Action to actually build it.