refactor: extract runtime artifact helpers

[chubes4]

Summary

• Extract Playground artifact bundle wiring into runtime-artifact-helpers.ts so index.ts no longer owns manifest, redaction, and log-format callback plumbing.
• Keep PlaygroundRuntime responsible for orchestration while preserving existing artifact collection behavior and public exports.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run artifact-bundle-verifier-smoke

Known blocked verification

• npm run artifact-contract-smoke currently hits the known upstream WordPress Playground port race (listen EADDRINUSE 127.0.0.1:<port>) and leaves the wrapper stuck until timeout. The upstream fix is tracked in Fix outbound proxy automatic port allocation WordPress/wordpress-playground#3722.
• npm run browser-probe-artifact-smoke also timed out before this cleanup on the same Playground-backed path.

AI assistance

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