Add Copilot multi-version .NET environment workflow

[Copilot]

Creates a GitHub Actions workflow that configures .NET 6, 8, and 10 SDKs for Copilot agent environment customization.

Workflow Configuration

• Multi-version setup: Single step installs .NET 6.0.428, 8.0.416, 10.0.101 using actions/setup-dotnet@v4 with pipe-delimited version syntax
• Performance: NuGet package caching via both actions/setup-dotnet built-in cache and explicit actions/cache@v4 targeting ~/.nuget/packages
• Tooling: Global dotnet-format installation for formatting capabilities
• Environment: Sets DOTNET_ROOT, DOTNET_CLI_TELEMETRY_OPTOUT, DOTNET_SKIP_FIRST_TIME_EXPERIENCE, DOTNET_NOLOGO
• Validation: Build step verifies SDK configuration, version listing steps confirm installations
• Security: Minimal contents: read permission
• Triggers: Manual dispatch, auto-run on workflow file changes

Job Naming

Job named copilot-setup-steps per Copilot agent environment customization docs for agent recognition.

Cache Strategy

Cache keys use hash of packages.lock.json, *.fsproj, *.csproj files with OS-specific prefix for accurate invalidation across dependency updates.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create Multi-Version .NET Setup Copilot Workflow</issue_title>
<issue_description># Create Multi-Version .NET Setup Copilot Workflow

Objective

Create a GitHub Actions workflow file that sets up a multi-version .NET environment for GitHub Copilot to work with .NET 6, 8, and 10 projects using best practices for security, performance, and maintainability.

Role

You are a DevOps engineer specializing in GitHub Actions and .NET CI/CD pipelines. Your task is to create a workflow that properly configures multiple .NET SDK versions following documented best practices.

Context

The workflow needs to be named copilot-setup-steps.yml and placed in .github/workflows/. It will automatically run when changed to validate the setup, and can be manually triggered through the Actions tab. The job must be named copilot-setup-steps to be recognized by Copilot. Docs: https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment

Requirements

• Create a GitHub Actions workflow that sets up .NET 6.0, 8.0, and 10.0 SDKs in a single step using multi-version syntax
• Use specific version numbers for reproducibility (not wildcards)
• Include NuGet package caching for performance
• Add .NET environment variables setup
• Install dotnet-format tool for code formatting
• Use minimum required permissions for security
• Add proper validation and build steps

Instructions

Create a workflow file with the following structure:

• Name: "Copilot Setup Steps for Multi-Version .NET"
• Triggers: workflow_dispatch, push, and pull_request on the workflow file path
• Job name: copilot-setup-steps
• Runner: ubuntu-latest
• Permissions: contents: read (minimum required)
• Steps: checkout with full history, setup .NET SDKs (multi-version), cache NuGet packages, restore dependencies, install dotnet-format, set environment variables, build validation

Expected Output

The workflow should create a .github/workflows/copilot-setup-steps.yml file that:

1. Sets up all three .NET SDK versions in a single step using proper multi-version syntax
2. Uses specific version numbers (e.g., 6.0.428, 8.0.416, 10.0.101)
3. Includes NuGet package caching for performance optimization
4. Installs dotnet-format tool globally
5. Sets up .NET environment variables (DOTNET_ROOT, DOTNET_CLI_TELEMETRY_OPTOUT)
6. Includes build validation to ensure setup works correctly
7. Uses proper GitHub Actions syntax with comments explaining each section
8. Follows security best practices with minimal permissions

Acceptance Criteria

• Workflow file is properly formatted YAML
• All three .NET versions are configured in a single setup step using multi-version syntax
• Specific version numbers are used (not wildcards)
• NuGet caching is implemented with proper cache keys
• dotnet-format tool installation step is included
• Environment variables for .NET are configured
• Build validation step is included to verify setup
• Job is named copilot-setup-steps
• Workflow triggers on appropriate events
• Permissions are set to minimum required (contents: read)
• Comments explain each section and security considerations
• Follows best practices for reproducibility and performance
  </issue_description>

<agent_instructions>Solve this issue</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Create Multi-Version .NET Setup Copilot Workflow #9

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