refactor: add exceptions.py module with structured exception hierarchy

- Create exceptions.py with 7 exception types: TwitterError, AuthenticationError,
  RateLimitError, NotFoundError, NetworkError, QueryIdError, TwitterAPIError
- Remove inline TwitterAPIError from client.py, import from exceptions module
- Replace RuntimeError('Cannot resolve queryId') with QueryIdError
- Replace RuntimeError('User not found') with NotFoundError
- Update test assertion for new TwitterAPIError message format

twitter_cli/exceptions.py

@@ -0,0 +1,45 @@
+"""Custom exceptions for twitter-cli.
+
+Provides a structured exception hierarchy for categorized error handling:
+- Authentication failures
+- API errors (rate-limit, not-found, forbidden)
+- Network errors
+- Query ID resolution failures
+
+Modeled after bilibili-cli/xiaohongshu-cli exception patterns.
+"""
+
+from __future__ import annotations
+
+
+class TwitterError(RuntimeError):
+    """Base exception for twitter-cli errors."""
+
+
+class AuthenticationError(TwitterError):
+    """Raised when cookies are missing, expired, or invalid."""
+
+
+class RateLimitError(TwitterError):
+    """Raised when Twitter rate limits the request (HTTP 429)."""
+
+
+class NotFoundError(TwitterError):
+    """Raised when a user or tweet is not found."""
+
+
+class NetworkError(TwitterError):
+    """Raised when upstream network requests fail."""
+
+
+class QueryIdError(TwitterError):
+    """Raised when a GraphQL queryId cannot be resolved."""
+
+
+class TwitterAPIError(TwitterError):
+    """Raised on non-OK Twitter API responses with HTTP status + message."""
+
+    def __init__(self, status_code: int, message: str):
+        self.status_code = status_code
+        self.message = message
+        super().__init__("Twitter API error (HTTP %d): %s" % (status_code, message))