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/observablehq.config.js

@@ -11,6 +11,7 @@ export default {
       pages: [
         { name: "Live Data", path: "/docs/live-data" },
         { name: "Decision Health", path: "/docs/decisions-kpi" },
+        { name: "Progress Log", path: "/docs/progress-log" },
       ],
     },
   ],