style: polish investigation workspace

- Summary panel: hide meaningless metrics (questions answered/remaining) until genuinely in progress; remove placeholder timestamps
- Understanding card: increased visual importance via larger heading, lighter border, more padding
- History section: reduced labels to brief forms ('History', 'Situation'), removed uppercase decorative labels from headings
- Investigation Map Preview: lighter borders, muted text, subtle background to signal provisional state
- Turn history cards: removed redundant subheadings ('Your answer', 'What changed') and divider lines
- Button label: 'Update situation' → 'Update'; padding consistent with design tokens
- Condition clarity fix: '!hasSelectedQuestion === false' → 'hasSelectedQuestion'
- Workspace polish section added to UX guidelines

components/investigation-summary-panel.jsx

@@ -132,40 +132,19 @@ function InvestigationSummaryPanel({ graph, selectedQuestion, result, updateStat
         </div>
       )}
 
-      {/* Questions */}
-      <div className="grid grid-cols-2 gap-4">
-        <div>
-          <span className="block text-xs text-gray-400">Questions answered</span>
-          <span className={`text-lg font-semibold ${colors.text}`}>{questionsAnswered}</span>
-        </div>
-        <div>
-          <span className="block text-xs text-gray-400">Still working on</span>
-          {/* TODO: avoid implying 1 unknown = 1 remaining question */}
-          {isInvestigating ? (
-            <span className={`text-lg font-semibold ${colors.text}`}>{questionsRemaining > 0 ? questionsRemaining + " items" : "—"}</span>
-          ) : (
-            <span className={`text-lg font-semibold ${colors.text}`}>—</span>
-          )}
-        </div>
-      </div>
-
-      {/* Timestamps */}
-      <div className="space-y-1 text-xs text-gray-400">
-        <div className="flex justify-between">
-          <span>Investigation started</span>
-          <span>{formatTimestamp(investigationStartTime)}</span>
-        </div>
-        <div className="flex justify-between">
-          <span>Last updated</span>
-          <span>{formatTimestamp(lastUpdatedAt)}</span>
-        </div>
-        {elapsedSeconds > 0 && (
-          <div className="flex justify-between">
-            <span>Elapsed since last update</span>
-            <span>{humaniseDuration(elapsedSeconds)}</span>
+      {/* Questions — hidden when no meaningful value to show */}
+      {isInvestigating && questionsRemaining > 0 && (
+        <div className="grid grid-cols-2 gap-4">
+          <div>
+            <span className="block text-xs text-gray-400">Questions answered</span>
+            <span className={`text-lg font-semibold ${colors.text}`}>{questionsAnswered}</span>
           </div>
-        )}
-      </div>
+          <div>
+            <span className="block text-xs text-gray-400">Still working on</span>
+            <span className={`text-lg font-semibold ${colors.text}`}>{questionsRemaining + " items"}</span>
+          </div>
+        </div>
+      )}
     </div>
   );
 }