feat: explore facilitated investigation workspace

Phase 4 UX exploration — workshop desk metaphor.

Workspace layout changes:
- Investigation Map promoted from preview to workspace artefact
- Understanding card given wider surface (lg:col-span-2)
- Grid shifts from equal-column to cognitive-weighted widths (lg:grid-cols-5)
- Mobile remains stacked; tablet simplifies naturally
- Desktop exploits wider working canvas

Layout structured by cognitive activity:
  Active workspace zone (Question + Response)
  Supporting workspace (Understanding, Map)
  Reference row (Situation, History)

components/investigation-map.jsx

@@ -84,10 +84,10 @@ export default function InvestigationMap({ turnCount = 0 }) {
   return (
     <div className="rounded-lg border border-gray-100 bg-gray-50/50 p-4" role="region" aria-label="Investigation map preview">
       <h2 className="mb-1 text-sm font-semibold tracking-tight text-gray-400">
-        Investigation Map Preview
+        Investigation Map
       </h2>
       <p className="mb-3 text-xs text-gray-400">
-        This preview shows where a future reasoning map may appear. Its final shape will emerge from the reasoning engine.
+        Active investigation topics and their status.
       </p>
 
       <div className="space-y-px border-t border-gray-100 pt-3" role="list" aria-label="Investigation topics">