Expose preview evidence refs

[chubes4]

Summary

• Add a wp-codebox/preview-evidence/v1 artifact to artifact bundles so downstream products can attach stable preview/session evidence to Conductor transfer-review/v1 records.
• Include stable browser Playground session/run refs, preview readiness progress, package/runtime component refs, and reviewer-safe preview URL refs.
• Omit loopback/local-only preview URL values from the new evidence artifact while retaining a local-only classification for accountability.

Trackers

• Closes Expose preview/session evidence refs for accountable transfer review #621
• Related: https://github.a8c.com/chubes4/conductor/issues/21
• Related: https://github.a8c.com/chubes4/studio-web/issues/406

Validation

• npm run build
• npm run artifact-contract-smoke
• npm run preview-options-contract-smoke
• npm run preview-port-smoke
• npm run preview-public-url-canonical-smoke
• npm run preview-response-body-smoke
• npm run browser-startup-progress-smoke
• npm run check

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the contract additions, artifact builder wiring, and focused smoke assertions; Chris remains responsible for review and testing.