[Task] Add a GitHub Actions release-artifact proof example

[ryan-wong-coder]

Background

Release artifacts are an easy-to-understand TrustDB use case. Maintainers should be able to generate and verify evidence in CI without reading server internals.

Scope

Add a minimal example workflow under examples or .github/examples that builds or downloads the TrustDB CLI, creates evidence for one release artifact, and documents how a recipient verifies it.

Acceptance criteria

• Uses only disposable demonstration keys.
• Pins a stable TrustDB release or clearly documents when a source build is used.
• Uploads the proof and public verification material as workflow artifacts.
• Includes a tamper-negative verification step.
• Includes a concise README explaining security boundaries.