Add initial Copilot instructions

[mbg]

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

Adds initial guidance for Copilot-assisted reviews by outlining which generated files and workflows to skip during code review.

• Introduces a Markdown file describing repository structure and review scope.
• Specifies directories and file patterns that are auto-generated and should be excluded from manual review.

Comments suppressed due to low confidence (1)

.github/copilot-instructions.md:5

• [nitpick] For clarity, change 'files in the .github/workflows directory that starts with two underscores' to 'files whose filenames start with two underscores in the .github/workflows directory'.

GitHub Actions workflows in the `.github/workflows` directory whose filenames start with two underscores (e.g. `__all-platform-bundle.yml`) are automatically generated using the `pr-checks/sync.sh` script from template files in the `pr-checks/checks` directory. Therefore, you do not need to review files in the `.github/workflows` directory that starts with two underscores. However, you should review changes to the `pr-checks` directory as well as workflows in the `.github/workflows` directory that do not start with underscores.

[esbena]

LGTM.

To me, this reads a lot like it has been written for and by a human. The word on the Promptcrafting street is that this might result in instructions that are harder to interpret for copilot.

Nevertheless, my position is that some tailored instructions are better than nothing. So LGTM.

[mbg]

Yes, we can always tweak this if it doesn't work well.