Extract PHP bootstrap helpers

[chubes4]

Summary

• Extract Playground PHP bootstrap/code loading helpers from the runtime class into php-bootstrap.ts.
• Keep runtime command behavior unchanged while reducing index.ts responsibility.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run phpunit-diagnostic-artifact-smoke
• npm run recipe-heavyweight-plugin-runtime-smoke
• npm run runtime-action-adapter-smoke
• npm run runtime-snapshot-restore-smoke
• npm run ability-login-blueprint-smoke

AI assistance

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