[Enhancement] Add Playground PR Preview button for Create Block Theme pull requests

Add a WordPress Playground PR Preview workflow so reviewers can test Create Block Theme pull requests directly in the browser without setting up a local WordPress environment.

Create Block Theme requires a build step before it can be previewed correctly, so this should use the build-artifact flow from the Playground PR Preview GitHub Action guide rather than the simple plugin-path: . flow.

Reference: https://wordpress.github.io/wordpress-playground/guides/github-action-pr-preview/

Problem

Reviewing UI or behavior changes currently requires contributors and reviewers to:

• check out the PR locally,
• install Node dependencies,
• run the plugin build,
• set up WordPress,
• activate the plugin,
• Navigate to the Create Block Theme admin page.

A Playground preview button would make PR review faster, especially for design, UI, onboarding, and Site Editor integration changes.

Proposed implementation

Add two GitHub workflows:

1. .github/workflows/pr-playground-preview.yml

   A read-only pull_request workflow that:

   • checks out the PR head,
   • sets up Node using .nvmrc,
   • runs npm ci,
   • runs npm run build,
   • packages a create-block-theme.zip artifact with a single top-level create-block-theme/ directory,
   • respects .distignore,
   • uploads the ZIP and PR metadata artifacts.

2. .github/workflows/pr-playground-preview-publish.yml

   A workflow_run publisher workflow that:

   • runs only after the build workflow succeeds,
   • does not check out or execute the pull request code,
   • validates the PR number and head SHA metadata,
   • exposes the built ZIP through WordPress/action-wp-playground-pr-preview,
   • generates a Playground Blueprint that installs and activates the built plugin,
   • appends the Playground preview button to the PR description.

The preview should land on:

/wp-admin/themes.php?page=create-block-theme-landing
Security considerations

Start with the Playground PR Preview GitHub Action guide, then inspect .nvmrc, .distignore, and the existing build commands. Add .github/workflows/pr-playground-preview.yml and pr-playground-preview-publish.yml, ensuring the built create-block-theme.zip and validated PR metadata reach the publisher safely. Done means a successful pull request gets a Playground preview button opening the specified Create Block Theme admin page.