Refactor Playground command routing out of index

[chubes4]

Summary

• Move the WordPress Playground command handler table into a dedicated command-router module.
• Keep index.ts as the runtime/public API surface while delegating command dispatch to the router.
• Preserve the existing playgroundRuntimeCommandIds() export for package consumers.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Refactored the command routing boundary, ran targeted verification, and drafted this PR summary. Chris remains responsible for review and merge.