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

tests/test_cli.py

@@ -104,10 +104,11 @@ def test_cli_commands_wrap_client_creation_errors(monkeypatch, args) -> None:
 
 
 def test_cli_user_error_yaml(monkeypatch) -> None:
+    from twitter_cli.exceptions import NotFoundError
     monkeypatch.setenv("OUTPUT", "auto")
     monkeypatch.setattr(
         "twitter_cli.cli._get_client",
-        lambda config=None, quiet=False: (_ for _ in ()).throw(RuntimeError("User not found")),
+        lambda config=None, quiet=False: (_ for _ in ()).throw(NotFoundError("User not found")),
     )
     runner = CliRunner()