feat: replace Unicode circle checkmarks with Primer octicon SVGs

[Copilot]

Task-list checkmarks were rendered as Unicode text characters (●/○), which don't scale cleanly or align with the Primer design system.

Changes

• scripts/lib/marked-setup.js — listitem renderer emits inline Primer octicon SVGs (check-circle-fill / circle, 16 px) instead of ●/○
• scripts/static/docs-checkboxes.js — applyItemState switches from marker.textContent to marker.innerHTML using two module-level SVG string constants
• scripts/lib/docs.css — strips text-character artifacts (border, border-radius, font-size, font-weight, line-height) from .task-list-item-marker; layout is now inline-flex; colour tokens (--fgColor-success, --fgColor-attention) drive icon colour via fill="currentColor"
• scripts/build-docs.test.js — assertion updated to match SVG marker output

<!-- before -->
<span class="task-list-item-marker is-pending" aria-hidden="true">○</span>

<!-- after -->
<span class="task-list-item-marker is-pending" aria-hidden="true">
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" width="16" height="16" fill="currentColor">
    <path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 …Z"></path>
  </svg>
</span>