ci(ai): add Copilot setup steps

[xperiandri]

Proposed Changes

Added a new GitHub Actions workflow at .github/workflows/copilot-setup-steps.yml to provide setup steps for Copilot agent runs. The workflow is triggered by manual dispatch, pushes that modify this workflow file, and pull requests that modify the same path. It defines a required copilot-setup-steps job on ubuntu-latest with minimal contents: read permissions. The job checks out the repository, installs .NET SDKs based on global.json plus versions 8.x, 9.x, and 10.x, then restores tools and solution dependencies using dotnet tool restore and dotnet restore FSharp.Control.R3.slnx.

Types of changes

What types of changes does your code introduce to FSharp.Control.R3?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally (not evidenced in diff/commits)
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)

[Copilot]

Pull request overview

Adds a dedicated GitHub Actions workflow to provide deterministic repository setup steps for GitHub Copilot agent runs, aligning the agent environment with the repo’s pinned .NET SDK/tooling and enabling easy validation of the setup steps.

Changes:

• Added .github/workflows/copilot-setup-steps.yml with a copilot-setup-steps job required for Copilot agent pre-run setup.
• Workflow checks out the repo, installs .NET SDKs (via global.json plus 8.x/9.x/10.x), and restores local tools + solution dependencies.