Create a Copilot prompt for release preparation steps

Create a Copilot prompt that can guide the release preparation work for cesium-native.

The prompt should help a human or agent prepare a release by:
- Gathering the current release branch, version, and scope.
- Checking for outstanding release blockers and required approvals.
- Verifying changelogs, version bumps, and release notes.
- Drafting any repeatable messages, checklists, or verification steps.
- Reducing manual copy/paste and making the preparation steps repeatable.

Acceptance criteria:
- The prompt is specific enough to be used as a repeatable release-prep helper.
- It clearly lists the steps it should perform and the inputs it needs.
- It focuses on preparation, not the final publish/release execution.

Use the issue's acceptance criteria as the starting checklist, and identify where a reusable Copilot prompt should live in the repository. Define the required release branch, version, scope, blockers, approvals, changelogs, version bumps, and release notes as inputs and steps; done means the prompt supports repeatable preparation without publishing the release.

Written by the indexing model from the issue text.