feat: add bookmark folders support (#30)

Add `twitter bookmarks folders` command to list bookmark folders
and `twitter bookmarks folders <id>` to fetch tweets from a specific
folder, with --since date filtering and pagination support.

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

twitter_cli/serialization.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import json
 from typing import Any, Dict, Iterable, List, Optional
 
-from .models import Author, Metrics, Tweet, TweetMedia, UserProfile
+from .models import Author, BookmarkFolder, Metrics, Tweet, TweetMedia, UserProfile
 from .timeutil import format_iso8601, format_local_time
 
 
@@ -175,6 +175,19 @@ def tweets_to_compact_json(tweets: Iterable[Tweet]) -> str:
     )
 
 
+def bookmark_folder_to_dict(folder: BookmarkFolder) -> Dict[str, Any]:
+    """Convert a BookmarkFolder dataclass into a JSON-safe dict."""
+    return {
+        "id": folder.id,
+        "name": folder.name,
+    }
+
+
+def bookmark_folders_to_data(folders: Iterable[BookmarkFolder]) -> List[Dict[str, Any]]:
+    """Serialize BookmarkFolder objects to Python dicts."""
+    return [bookmark_folder_to_dict(f) for f in folders]
+
+
 def user_profile_to_dict(user: UserProfile) -> Dict[str, Any]:
     """Convert a UserProfile dataclass into a JSON-safe dict."""
     return {