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/models.py

@@ -54,6 +54,12 @@ class Tweet:
     article_text: Optional[str] = None
 
 
+@dataclass
+class BookmarkFolder:
+    id: str
+    name: str
+
+
 @dataclass
 class UserProfile:
     id: str