refactor: isolate Playground runtime orchestration

[chubes4]

Summary

• Moves the Playground runtime backend and orchestration class out of packages/runtime-playground/src/index.ts into playground-runtime.ts.
• Leaves index.ts as a thin public package entrypoint that re-exports the stable backend factory and command id helper.
• Keeps command runners, artifacts, snapshots, preview, and observation helper boundaries unchanged.

Closes #345.

Testing

• npm run build
• npm run command-registry-smoke
• npm run runtime-episode-smoke
• npm run runtime-action-adapter-smoke
• npm run artifact-contract-smoke

Notes

• npm run runtime-snapshot-restore-smoke was attempted twice and hit the known upstream WordPress Playground port race (EADDRINUSE on 127.0.0.1), matching [CLI] Avoid port conflicts when starting PHP-WASM network proxies WordPress/wordpress-playground#3722.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Inspected the runtime-playground boundaries, planned the final move-only refactor, drafted the file extraction, and ran targeted verification; Chris remains responsible for review and merge.