Enable interactive workshop checklist checkboxes on GitHub Pages

[Copilot]

Workshop task-list checkboxes rendered in docs/step.html were non-interactive on the deployed GitHub Pages site, so learners could not track progress in-page. This change makes those checkboxes clickable and preserves their state across refreshes and step navigation.

• Checkbox interactivity in rendered markdown

  • After marked.parse(md), initialize all input[type="checkbox"] inside #markdown-content.
  • Explicitly enable checkboxes so GitHub-style rendered task lists are user-clickable.

• Per-step persistence model

  • Add local storage persistence under agent-lab-checkboxes-v1.
  • Store state by workshop step ID (00-overview, 01-setup, etc.) and checkbox index.
  • Rehydrate checked state on content load for the active step.

• State management hardening

  • Add safe read/write wrappers for storage parsing/failures.
  • Debounce writes to avoid excessive storage churn during rapid toggling.
  • Guard initialization when the markdown container is unavailable.

<!-- docs/step.html -->
document.getElementById('markdown-content').innerHTML = marked.parse(md);
initializeTaskListCheckboxes(step.id); // enable + restore per-step checkbox state