Two bugs fixed:
1. Banner not shown: TUI default output was a plain welcome text
instead of the actual GyanDoot banner. Fixed by using
banner::banner_plain() as initial TUI output content.
2. No agent responses: TUI was a standalone shell with no connection
to the agent loop. Now uses mpsc channels:
- user_tx: TUI → agent (user sends messages)
- event_rx: agent → TUI (agent sends responses + telemetry)
- TuiEvent::Response / Status / Telemetry enum for structured
communication
- tokio::spawn runs agent in background processing messages
- poll_events() in render loop drains channel every frame tick
Also updated cmd_tui() in main to create channels + spawn agent.