Refactor playground command error helpers

[chubes4]

Summary

• Extract Playground command response/error helpers into playground-command-errors.ts.
• Keep runtime execution behavior unchanged while moving failure shaping and core-phpunit diagnostic parsing out of index.ts.

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 core-phpunit-command-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.