Add top-of-conversation autotest status indicator with jump-to-block behavior

[Copilot]

Long PR timelines make it costly to find autotest results after reversing comments. This change surfaces autotest state near the PR description area and provides a one-click jump to the underlying timeline entry.

• Top-level autotest signal on Conversation pages

  • Adds a lightweight status button (#pr-reverse-comments-autotest-status) above the timeline container when an autotest timeline item is present.
  • Scopes rendering to PR conversation pages only (not commits/files/checks).

• Status derivation and visual state

  • Parses autotest entry text to map state to concise labels/colors:
    • ✓ Autotest passing
    • ✗ Autotest failing
    • • Autotest running
    • fallback • Autotest status
  • Keeps styling intentionally minimal and GitHub-theme-friendly.

• Navigation and lifecycle behavior

  • Clicking the indicator scrolls to the current autotest timeline item and briefly highlights it.
  • Integrates with existing soft-nav/rebind flow so the indicator is inserted, updated, or removed as DOM/routes change.

• Docs

  • Updates README usage text to describe the new top-of-page autotest indicator and click-to-jump behavior.

const target = getAutotestTimelineItem();
const state = getAutotestState(target.textContent || "");
indicator.textContent = state.label;
indicator.style.border = `1px solid ${state.color}`;

indicator.addEventListener("click", () => {
  const freshTarget = getAutotestTimelineItem();
  if (!freshTarget) return;
  freshTarget.scrollIntoView({ behavior: "smooth", block: "center" });
});