dashboard: add progress log documentation and ? button on page heading

- src/docs/progress-log.md: covers append-only constitution §5 guarantee,
  event structure (all fields), standard + convention event types, session
  protocol, MCP and curl usage, filters, and the 30-day volume chart
- progress.md: withDocHelp applied to #observablehq-main h1 → ? button
  appears on hover over the 'Progress Log' page heading
- observablehq.config.js: Progress Log added to Reference nav section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dashboard/src/progress.md

@@ -44,6 +44,9 @@ const _liveEl = html`<div class="live-indicator">
 </div>`;
 withDocHelp(_liveEl, "/docs/live-data");
 injectTocTop("live-indicator", _liveEl);
+
+const _h1 = document.querySelector("#observablehq-main h1");
+if (_h1) { _h1.style.position = "relative"; withDocHelp(_h1, "/docs/progress-log"); }
 ```
 
 ## Event Volume (Last 30 Days)