Add browser probe checkpoints

[chubes4]

Summary

• Adds generic wordpress.browser-probe checkpoint support via window.__wpCodeboxProbeCheckpoint(name, metadata = {}) and files/browser/checkpoints.jsonl.
• Adds optional repeat=<n> and reset-between=none|reload|new-page controls with per-iteration checkpoint and memory summaries.
• Updates command metadata, recipe validation, README docs, and browser probe smoke coverage.

Closes #303.

Validation

• npm run build
• npm run command-registry-smoke
• npm run recipe-dry-run-smoke
• npm run browser-probe-artifact-smoke

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the browser-probe checkpoint/repeat contract, docs, validation, smoke assertions, and targeted verification; Chris remains responsible for review and merge.