feat: add folder support for bookmarks command

src/lib/extract-bookmark-folder-id.ts

@@ -0,0 +1,20 @@
+/**
+ * Extract bookmark folder (collection) ID from an X/Twitter bookmarks URL.
+ */
+const BOOKMARK_FOLDER_URL_REGEX = /(?:twitter\.com|x\.com)\/i\/bookmarks\/(\d+)/i;
+const BOOKMARK_FOLDER_ID_REGEX = /^\d{5,}$/;
+
+export function extractBookmarkFolderId(input: string): string | null {
+  const trimmed = input.trim();
+  if (!trimmed) {
+    return null;
+  }
+  const urlMatch = BOOKMARK_FOLDER_URL_REGEX.exec(trimmed);
+  if (urlMatch) {
+    return urlMatch[1];
+  }
+  if (BOOKMARK_FOLDER_ID_REGEX.test(trimmed)) {
+    return trimmed;
+  }
+  return null;
+}