Add thread/conversation support

[sujito00]

Problem

There's no way to fetch a full conversation thread starting from a tweet. replies only fetches direct replies to a single tweet, not the full thread context (parent chain + replies).

Proposed solution

Add a thread command that:

1. Given a tweet ID, walks up the in_reply_to_status_id chain to find the thread root
2. Fetches all tweets in the thread from root to leaf
3. Displays them in order

x-reader thread 1234567890
x-reader thread https://x.com/user/status/1234567890 --format json

API approach

The TweetDetail GraphQL endpoint already returns conversation threads via threaded_conversation_with_injections_v2. The data is likely already in the response from getTweet(), just not parsed.