Add two new commands to list users:
- `bird following` - list users that you (or another user) follow
- `bird followers` - list users that follow you (or another user)
Both commands support:
- `--user <userId>` to query another user's following/followers
- `-n <count>` to limit results
- `--json` for structured output
Uses Twitter's GraphQL Following/Followers endpoints with proper
feature flags and query ID rotation support.
