Backend changes:
- Add CreatedAt() to Session interface
- Implement CreatedAt() in tmux.Session (field already existed)
- Sort Manager.List() by creation time descending
Benefits:
- Deterministic ordering (no more random map iteration)
- Newest sessions appear first (most useful)
- All clients benefit immediately
- head -1 now reliably gets most recent session
README:
- Update example to use head -1 for newest session
- Document that list is sorted newest first
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