Support multiple blueprint preview links

[akirk]

What

• Update the Playground preview comment workflow to include every changed blueprint.json file in a PR.
• Render one Playground link per blueprint and update the existing marked comment.
• Build raw blueprint URLs from the PR head repository and SHA so fork PRs work correctly.

Testing

• Parsed the workflow YAML successfully.
• Checked the embedded github-script syntax with Node.
• Simulated Add Theme Review Environment #181 shaped data and confirmed two preview links are rendered.

Example output

Blueprint previews

• Open hello-blueprints in WordPress Playground
• Open theme-review-environment in WordPress Playground