refactor: unify exception handling, add ISO 8601 time, dedup commands, expand tests

- Replace _error_code_for_message() string matching with error_code attribute on exception classes
- Add error_code to all TwitterError subclasses (AuthenticationError, RateLimitError, etc.)
- Add InvalidInputError exception class
- TwitterAPIError derives error_code from HTTP status code automatically
- auth.py: use AuthenticationError instead of RuntimeError
- cli.py: catch (TwitterError, RuntimeError) for backward compat
- Extract _fetch_and_display_users() to deduplicate followers/following commands
- Add format_iso8601() to timeutil.py
- Add createdAtISO field to tweet and user profile serialization
- New test files: test_output.py, test_cache.py, test_timeutil.py
- Expand test_filter.py (topN, score mode, custom weights, empty input)
- Tests: 152 → 194 unit tests, all passing

twitter_cli/serialization.py

@@ -6,7 +6,7 @@ import json
 from typing import Any, Dict, Iterable, List, Optional
 
 from .models import Author, Metrics, Tweet, TweetMedia, UserProfile
-from .timeutil import format_local_time
+from .timeutil import format_iso8601, format_local_time
 
 
 def tweet_to_dict(tweet: Tweet) -> Dict[str, Any]:
@@ -31,6 +31,7 @@ def tweet_to_dict(tweet: Tweet) -> Dict[str, Any]:
         },
         "createdAt": tweet.created_at,
         "createdAtLocal": format_local_time(tweet.created_at),
+        "createdAtISO": format_iso8601(tweet.created_at),
         "media": [
             {
                 "type": media.type,
@@ -190,6 +191,7 @@ def user_profile_to_dict(user: UserProfile) -> Dict[str, Any]:
         "verified": user.verified,
         "profileImageUrl": user.profile_image_url,
         "createdAt": user.created_at,
+        "createdAtISO": format_iso8601(user.created_at),
     }