Root cause: `xmaster replies <id>` searched `conversation_id:<id>` but
when the target tweet is itself a reply, its conversation_id equals the
thread root, not its own ID. So the search returned nothing.

Fix:
1. Fetch the target tweet first to get its actual conversation_id
2. Search the full conversation by that conversation_id
3. Filter results to only direct replies to the target tweet using
   the referenced_tweets field (type=replied_to, id=target)

Also added conversation_id and referenced_tweets to TweetData struct
and tweet_fields API parameter.

Fixed test: schedule_list_empty now expects failure (exit 1) after
the v1.2.6 exit code hardening.