feat: detect subscriber-only tweets via tweetInterstitial (#33)

Extract visibility metadata from TweetWithVisibilityResults wrapper
before unwrapping. Adds is_subscriber_only field to Tweet model,
with full serialization roundtrip and test coverage.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

twitter_cli/models.py

@@ -52,6 +52,7 @@ class Tweet:
     score: Optional[float] = None
     article_title: Optional[str] = None
     article_text: Optional[str] = None
+    is_subscriber_only: bool = False
 
 
 @dataclass