Give Copilot code review a minimal dedicated setup

[automated]

## Summary

Aspire does not have `.github/workflows/copilot-code-review.yml`, so Copilot code review falls back to the full coding-agent setup. That setup restores the solution, installs tools, and installs multiple SDKs before review begins. Recent observed Aspire review sessions did not invoke `dotnet`, `build.sh`, or the restored solution.

## Scope

- Add the supported `.github/workflows/copilot-code-review.yml` workflow with the required `copilot-setup-steps` job.
- Keep the current review runner selection unchanged for the first experiment so the setup-content change can be measured independently.
- Use the smallest valid setup. Do not restore the solution or install Verify, `gh-aw`, or additional SDKs unless a demonstrated code-review requirement needs them.
- Add an appropriate manual or change-validation trigger if supported for this workflow.
- Do not modify the coding-agent setup workflow in this PR.

## Acceptance criteria

- The workflow follows GitHub's current Copilot code-review workflow contract and is valid GitHub Actions YAML.
- Code review no longer falls back to `.github/workflows/copilot-setup-steps.yml` after this reaches the default branch.
- The PR identifies every retained setup step and the review requirement that justifies it.
- Before-and-after review measurements are collected using at least five fresh runs of the same representative PR set, or the PR clearly marks measurement as a required post-merge checkpoint if GitHub cannot exercise the new setup before merge.
- Review findings are compared for material quality regressions, not only elapsed time.