Add Copilot instructions for release PRs

[mbg]

I wrote some notes for this and then let Copilot edit them.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[henrymercer]

This seems contradictory to the instruction above not to review these PRs — consider combining these instructions?

[Copilot]

Pull Request Overview

This PR adds Copilot instructions for handling release PRs to ensure proper review coverage. It establishes clear guidelines for identifying release PRs and specifies which files should be reviewed during the release process.

Key changes:

• Adds a "Generated code" section header to organize existing content
• Defines criteria for identifying release PRs using title, branch, and target patterns
• Specifies that only CHANGELOG.md and package.json should be reviewed in release PRs

Comments suppressed due to low confidence (1)

.github/copilot-instructions.md:7

• Grammar inconsistency: 'start with two underscores' should be 'that start with two underscores' for consistency with the first usage in the sentence.

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 start 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.