Extract check artifact helpers

[chubes4]

Summary

• Extract Plugin Check and Theme Check artifact writing into check-artifacts.ts.
• Move check artifact manifest and redaction helpers out of the Playground runtime class.
• Keep index.ts focused on running check commands while preserving existing artifact metadata behavior.

Testing

• npm run build
• npm run plugin-check-normalization-smoke
• npm run theme-check-normalization-smoke
• npm run artifact-contract-smoke
• npm run command-registry-smoke
• npm run package-distribution-smoke

AI assistance

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