Build a terminal UI for navigating and interacting with the agent graph.
Features:
- Visual tree/DAG display of conversation branches
- Vim-style navigation (j/k/h/l for siblings/parent/child)
- Compose new messages at current position
- See node status (idle/running/done/error)
- Fork/join operations via keyboard shortcuts
See: Omni/Agent/Graph/DESIGN.md (TUI section)