Copilot instructions

[rpahl]

This pull request introduces improvements to the R environment setup for CI workflows and documents conventions for Copilot contributions. The most significant change is the creation of a reusable GitHub Action for setting up the R check environment, which streamlines and centralizes environment configuration across workflows. Additional updates include the addition of Copilot instructions, modifications to CI workflow steps to use the new action, and the introduction of a dedicated workflow for validating setup steps.

R Environment Setup Improvements

• Added a new composite GitHub Action (.github/actions/r-check-env/action.yaml) to set up the common R environment, including Pandoc, system libraries, R, and renv, for use in CI and Copilot workflows.
• Refactored .github/workflows/ci.yaml to use the new r-check-env action, replacing repeated setup steps for Pandoc, system libraries, R, and renv with a single reusable step.

Copilot and Workflow Enhancements

• Added .github/copilot-instructions.md to document Copilot goals, stack, code conventions, test practices, and commit message standards for contributors.
• Introduced .github/workflows/copilot-setup-steps.yml, a dedicated workflow to validate the R environment setup steps, triggered on changes to the setup workflow or manually via the Actions tab.

Minor Workflow Adjustments

• Minor formatting and spacing improvements in .github/workflows/ci.yaml for clarity and organization. [1] [2]