Refactor playground CLI runner helpers

[chubes4]

Summary

• Extract WordPress Playground CLI startup mechanics into playground-cli-runner.ts.
• Keep runtime lazy boot/caching in index.ts while moving the process.exit guard, dynamic CLI typing, blueprint wiring, preview-port check, and preview proxy setup behind one helper.

Testing

• npm install (fresh worktree dependency install; prepare build passed)
• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run preview-port-smoke
• npm run runtime-episode-smoke
• npm run boot-preview-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the focused extraction and ran verification; Chris remains responsible for review and merge.