Add copilot-setup-steps.yml for .NET 10.0.100 RC1 SDK preinstallation

[Copilot]

Summary

This PR adds a copilot-setup-steps.yml file to preinstall the .NET 10.0.100 RC1 SDK in the GitHub Copilot Coding Agent environment, following the official documentation for customizing the agent environment.

Changes

Added .github/copilot-setup-steps.yml that:

1. Downloads and installs the .NET SDK using the official Microsoft installation script

   • Installs the exact version required by this repository: 10.0.100-rc.1.25451.107 (as specified in global.json)
   • Installs to $HOME/.dotnet to avoid permission issues
   • Adds the SDK to $GITHUB_PATH to make it available for all subsequent Copilot agent steps

2. Verifies the installation by running dotnet --version to confirm the SDK is properly installed and accessible

Why This Matters

The repository requires .NET 10.0.100 RC1 to build and test successfully. Without this preinstallation, the GitHub Copilot Coding Agent would encounter build failures when working on this repository, as the agent environment doesn't include this preview SDK by default.

With this setup file in place, the Copilot Coding Agent will automatically have the correct .NET SDK version available, enabling it to:

• Build the solution successfully
• Run tests to validate changes
• Provide accurate code suggestions that work with .NET 10 features

Testing

• ✅ Verified YAML syntax is valid
• ✅ Confirmed the installation script URL is accessible
• ✅ Validated the SDK version matches the repository's global.json requirement
• ✅ Follows the GitHub Copilot setup steps specification format

Original prompt

Add a copilot-setup-steps.yml file for GitHub Copilot Coding Agent. The workflow should preinstall the .NET 10.0.100 RC1 SDK in the Copilot agent environment, following the official documentation: https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment#preinstalling-tools-or-dependencies-in-copilots-environment. The SDK can be downloaded from https://dotnet.microsoft.com/en-us/download/dotnet/thank-you/sdk-10.0.100-rc.1-linux-x64-binaries and should be installed and available in the environment for subsequent agent steps.

This pull request was created as a result of the following prompt from Copilot chat.

Add a copilot-setup-steps.yml file for GitHub Copilot Coding Agent. The workflow should preinstall the .NET 10.0.100 RC1 SDK in the Copilot agent environment, following the official documentation: https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment#preinstalling-tools-or-dependencies-in-copilots-environment. The SDK can be downloaded from https://dotnet.microsoft.com/en-us/download/dotnet/thank-you/sdk-10.0.100-rc.1-linux-x64-binaries and should be installed and available in the environment for subsequent agent steps.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Meir017]

Should probably use something like this https://github.com/dotnet/aspire/blob/911e1f62c0caa3fae683218f24ef0cd0be3e1efb/.github/workflows/copilot-setup-steps.yml#L35

[Meir017]

or maybe something like this:

- uses: actions/setup-dotnet@v5
  with:
    global-json-file: csharp/global.json

https://github.com/actions/setup-dotnet?tab=readme-ov-file#using-the-global-json-file-input

[stephentoub]

@copilot , can you update using the global-json-file approach?

[stephentoub]

@copilot, please see the previous comment about global-json-file and update to use that approach.

[eiriktsarpalis]

Should we create an issue to undo these instructions once .NET 10 is GA?

[halter73]

or maybe something like this:

• uses: actions/setup-dotnet@v5
  with:
  global-json-file: csharp/global.json
  actions/setup-dotnet#using-the-global-json-file-input

That's cool. We should also update all the workflows I updated in #871 to use global-json-file too. I wonder if we could use that in addition to dotnet-version for 9.0.x.

If we do this, I don't think we'd need an issue specifically for updating instructions since all we'd need to update is the global.json.

[stephentoub]

@copilot, can you update using the global-json-file approach?

[stephentoub]

copilot's gotten stuck. Trying a new PR.
#877