Add browser review bridge for sandbox artifacts

[chubes4]

Summary

• Add a reusable core browser review bridge with Codebox-owned metadata and decision schemas.
• Provide structured wp-codebox:artifact-review-decision postMessage helpers plus an optional generic overlay with caller-supplied labels.
• Document how parent hosts can feed approval decisions into stage-artifact-apply before apply-approved-artifact resolves apply-back.

Closes #439.

Verification

• npm run build
• npm run browser-review-bridge-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted and implemented the browser review bridge, smoke coverage, and README usage docs; Chris remains responsible for review and testing.