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/commands/__init__.py

@@ -0,0 +1,7 @@
+"""CLI command sub-modules for twitter-cli.
+
+Commands are split into three groups:
+  - read: feed, bookmarks, search, tweet, article, show, list, favorites
+  - write: post, reply, quote, delete, like/unlike, retweet/unretweet, bookmark/unbookmark
+  - user: user, user-posts, likes, followers, following, whoami, status, follow/unfollow
+"""