test(cli): validate mentions handle

src/lib/normalize-handle.ts

@@ -11,3 +11,20 @@ export function normalizeHandle(input?: string | null): string | null {
 
   return handle;
 }
+
+export function mentionsQueryFromUserOption(userOption?: string | null): {
+  query: string | null;
+  error: string | null;
+} {
+  if (typeof userOption === 'undefined') return { query: null, error: null };
+
+  const handle = normalizeHandle(userOption);
+  if (!handle) {
+    return {
+      query: null,
+      error: 'Invalid --user handle. Expected something like @steipete (letters, digits, underscore; max 15).',
+    };
+  }
+
+  return { query: `@${handle}`, error: null };
+}