Refactor browser probe helpers out of runtime index

[chubes4]

Summary

• Move browser-probe navigation, viewport capture, replayability classification, checkpoint collection, and probe memory/performance artifact helpers into browser-probe.ts.
• Keep runBrowserProbe() as the orchestration and artifact-writing boundary while delegating probe-specific helper behavior.
• Preserve existing browser probe artifact output shapes and metric collection behavior.

Testing

• npm run build
• npm run command-registry-smoke
• npm run package-distribution-smoke
• npm run browser-actions-artifact-smoke
• npm run browser-probe-artifact-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Identified and extracted the browser probe helper boundary, ran targeted verification, and drafted this PR summary. Chris remains responsible for review and merge.