Promote browser probe metrics through bench output

[chubes4]

Summary

• Add generic performance/memory capture support to wordpress.browser-probe, including raw files/browser/performance.json and files/browser/checkpoints.jsonl artifacts.
• Promote captured browser metrics into wordpress.bench scenario metrics using generic browser_* names when prior browser probes exist in the same runtime.
• Add smoke coverage for browser metric promotion and update command/docs references.

Closes #304.

Tests

• npm run build
• npm run recipe-browser-bench-metrics-smoke
• npm run recipe-bench-smoke
• npm run browser-probe-artifact-smoke
• npm run discovery-command-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the implementation, smoke coverage, documentation updates, and ran targeted verification; Chris remains responsible for review and merge.