Add copilot-setup-steps.yml to preinstall .NET 10 SDK in GitHub Copilot environment (#877)

* Initial plan

* Add copilot-setup-steps.yml for .NET 10 SDK preinstallation

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Use semantic version tag for setup-dotnet action

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Add dotnet --info step to verify SDK installation

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

.github/copilot-setup-steps.yml

@@ -0,0 +1,6 @@
+name: Setup .NET SDK
+steps:
+  - uses: actions/setup-dotnet@v5
+    with:
+      global-json-file: global.json
+  - run: dotnet --info