Extract browser command runners

[chubes4]

Summary

• Extract wordpress.browser-probe and wordpress.browser-actions execution into browser-command-runners.ts.
• Keep the Playground runtime class responsible for booting the server and storing produced browser artifacts.
• Move browser capture validation, Playwright orchestration, artifact file writing, and command output shaping out of index.ts.

Testing

• npm run build
• npm run browser-probe-artifact-smoke
• npm run browser-actions-artifact-smoke
• npm run browser-interaction-script-validation-smoke
• npm run recipe-browser-bench-metrics-smoke
• npm run command-registry-smoke
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the browser command runner extraction, ran verification, and prepared the PR summary for Chris to review.