fix(cli): harden /btw command error handling and type safety

- Add null/undefined guard in formatBtwError to avoid "null"/"undefined" strings
- Add type guard for btw property in HistoryItemDisplay to prevent crash
- Extract isBtwCommand regex to module-level constant and simplify with [/?]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/ui/components/HistoryItemDisplay.tsx

@@ -227,7 +227,9 @@ const HistoryItemDisplayComponent: React.FC<HistoryItemDisplayProps> = ({
       {itemForDisplay.type === 'insight_progress' && (
         <InsightProgressMessage progress={itemForDisplay.progress} />
       )}
-      {itemForDisplay.type === 'btw' && <BtwMessage btw={itemForDisplay.btw} />}
+      {itemForDisplay.type === 'btw' && itemForDisplay.btw && (
+        <BtwMessage btw={itemForDisplay.btw} />
+      )}
     </Box>
   );
 };