feat: add whoami, reply, quote, follow/unfollow commands and --compact mode
- whoami: fetch current authenticated user profile
- reply <id> <text>: standalone reply command
- quote <id> <text>: quote-tweet command
- follow/unfollow <handle>: follow/unfollow users
- --compact/-c: global flag for LLM-friendly minimal JSON output
- client.py: add fetch_me, quote_tweet, follow_user, unfollow_user
- serialization.py: add tweet_to_compact_dict, tweets_to_compact_json
- 7 new tests (82 total, all passing)