Add a reminder to the PR template that this is a public repo

[mbg]

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.

[Copilot]

Pull Request Overview

This PR adds a reminder comment to the pull request template to notify GitHub staff that the repository is public. This helps ensure proper consideration of sensitive information when creating pull requests.

• Adds a single HTML comment at the top of the PR template as a visibility reminder