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/commands/btwCommand.ts

@@ -21,7 +21,8 @@ function makeBtwPromptId(sessionId: string): string {
 
 function formatBtwError(error: unknown): string {
   return t('Failed to answer btw question: {{error}}', {
-    error: error instanceof Error ? error.message : String(error),
+    error:
+      error instanceof Error ? error.message : String(error || 'Unknown error'),
   });
 }