Add Playground PR preview workflows

[fellyph]

Summary

• Add fork-safe WordPress Playground PR Preview workflows for Two Factor pull requests.
• Build the plugin with the existing npm run build process, package dist/ as a slug-shaped two-factor.zip, and publish a Playground preview button after the read-only build workflow succeeds.
• Use a custom Blueprint so the preview logs in and opens the Two Factor settings page.

Why

Two Factor needs generated preview files because the release build copies the QR code dependency from node_modules into dist/. The two-workflow setup keeps pull request code in a read-only build workflow, then lets the trusted workflow_run publish workflow expose the ZIP and update the PR description.

Testing

• Parsed both workflow files as YAML.
• Parsed the embedded Blueprint JSON.
• Ran npm ci.
• Ran the configured build/ZIP steps and verified build/two-factor.zip extracts to a single two-factor/ directory.
• Ran npm run lint:js.
• Ran npm run lint:css.
• Ran composer validate --strict.
• Ran composer lint-compat.
• Ran composer lint-phpstan.

PHPUnit was not run locally because wp-env could not connect to Docker in this environment.