Replace String() calls with .toString()/.toLocaleString() per ast-grep rule

- Add formatError() helper in CLI to safely handle unknown error types
- Add uniqueTestId() helper for generating unique test identifiers
- Replace String(id) with id.toString() for database ID conversions
- Replace String(n) with n.toLocaleString() for user-facing number formatting
- Fix TypeScript errors in test files (undefined checks, unused variables)
- Update lint commands to include ast-grep scanning

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/common/src/format-date.ts

@@ -95,11 +95,11 @@ export function formatRelativeDate(
     return "Yesterday";
   }
   if (diffDays < 7) {
-    return `${String(diffDays)} days ago`;
+    return `${diffDays.toLocaleString()} days ago`;
   }
   if (diffDays < 30) {
     const weeks = Math.floor(diffDays / 7);
-    return weeks === 1 ? "1 week ago" : `${String(weeks)} weeks ago`;
+    return weeks === 1 ? "1 week ago" : `${weeks.toLocaleString()} weeks ago`;
   }
 
   // For older dates, show the actual date