Fix TUI shutdown: send /shutdown message and handle in agent loop

When the TUI quits (Ctrl+D twice), it now:
1. Sends a "/shutdown" message through the channel before closing
2. Explicitly drops msg_tx to ensure channel closure

The agent loop now:
1. Returns Option<String> from handle_message (None = shutdown)
2. Handles /quit, /exit, /shutdown commands by returning None
3. Breaks out of the main loop on shutdown signal
4. Lists /quit in help menu

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/channels/cli/events.rs

@@ -26,8 +26,13 @@ pub fn run_event_loop(
         // Render
         terminal.draw(|f| render::render(f, app))?;
 
-        // Check for quit
+        // Check for quit - send shutdown signal and exit
         if app.should_quit {
+            // Send a shutdown message so the agent loop knows to exit
+            let shutdown_msg = IncomingMessage::new("tui", "system", "/shutdown");
+            let _ = msg_tx.blocking_send(shutdown_msg);
+            // Explicitly drop to close the channel
+            drop(msg_tx);
             return Ok(());
         }