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/timeutil.py

@@ -69,3 +69,14 @@ def format_relative_time(created_at: str) -> str:
         return "%dmo ago" % months
     years = days // 365
     return "%dy ago" % years
+
+
+def format_iso8601(created_at: str) -> str:
+    """Convert Twitter timestamp to ISO 8601 format.
+
+    Returns e.g. "2026-03-08T12:00:00+00:00" or the original string on failure.
+    """
+    dt = _parse_twitter_time(created_at)
+    if dt is None:
+        return created_at
+    return dt.isoformat()