Expose browser metrics from artifact bundles

[chubes4]

Summary

• Add wp-codebox artifacts browser-metrics --bundle <dir> --json so parent bench consumers can read browser metrics without parsing WP Codebox browser artifact internals.
• Return an explicit no-browser-metrics result for bundles without browser artifacts.
• Extend browser bench metrics smoke coverage for recipe-run bench promotion plus artifact-bundle metrics extraction with and without browser artifacts.

Testing

• npm run build
• npm run recipe-browser-bench-metrics-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted and verified the CLI command, shared browser metrics helper, and smoke coverage; Chris remains responsible for review and merge.