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/exceptions.py

@@ -6,7 +6,7 @@ Provides a structured exception hierarchy for categorized error handling:
 - Network errors
 - Query ID resolution failures
 
-Modeled after bilibili-cli/xiaohongshu-cli exception patterns.
+Each exception carries an `error_code` attribute for structured output.
 """
 
 from __future__ import annotations
@@ -15,30 +15,50 @@ from __future__ import annotations
 class TwitterError(RuntimeError):
     """Base exception for twitter-cli errors."""
 
+    error_code: str = "api_error"
+
 
 class AuthenticationError(TwitterError):
     """Raised when cookies are missing, expired, or invalid."""
 
+    error_code = "not_authenticated"
+
 
 class RateLimitError(TwitterError):
     """Raised when Twitter rate limits the request (HTTP 429)."""
 
+    error_code = "rate_limited"
+
 
 class NotFoundError(TwitterError):
     """Raised when a user or tweet is not found."""
 
+    error_code = "not_found"
+
 
 class NetworkError(TwitterError):
     """Raised when upstream network requests fail."""
 
+    error_code = "network_error"
+
 
 class QueryIdError(TwitterError):
     """Raised when a GraphQL queryId cannot be resolved."""
 
+    error_code = "query_id_error"
+
 
 class MediaUploadError(TwitterError):
     """Raised when media upload fails (file not found, too large, unsupported format, API error)."""
 
+    error_code = "media_upload_error"
+
+
+class InvalidInputError(TwitterError):
+    """Raised when user input is invalid (bad tweet ID, invalid options, etc.)."""
+
+    error_code = "invalid_input"
+
 
 class TwitterAPIError(TwitterError):
     """Raised on non-OK Twitter API responses with HTTP status + message."""
@@ -46,4 +66,13 @@ class TwitterAPIError(TwitterError):
     def __init__(self, status_code: int, message: str):
         self.status_code = status_code
         self.message = message
+        # Derive error_code from HTTP status
+        if status_code in (401, 403):
+            self.error_code = "not_authenticated"
+        elif status_code == 429:
+            self.error_code = "rate_limited"
+        elif status_code == 404:
+            self.error_code = "not_found"
+        else:
+            self.error_code = "api_error"
         super().__init__("Twitter API error (HTTP %d): %s" % (status_code, message))