Extract mounted artifact capture

[chubes4]

Summary

• Extract mounted file capture and baseline diff capture from the Playground runtime class into mounted-artifact-capture.ts.
• Keep index.ts responsible for wiring artifact bundle collection while the new module owns recursive mount walking, skipped-file limits, redaction, and patch collection.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run artifact-contract-smoke
• npm run artifact-bundle-verifier-smoke
• npm run artifact-patch-git-apply-smoke

AI assistance

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