feat(ui): implement graceful quit via /quit command and ctrl+c in AppModel

Handle /quit slash command (and aliases /q, /exit) in submitMsg handler by
returning tea.Quit before forwarding to the app layer. Add TODO comment in
cmd/root.go marking where defer appInstance.Close() belongs once app.App
exists (TAS-18).

cmd/root.go

@@ -459,6 +459,9 @@ func runNormalMode(ctx context.Context) error {
 		return fmt.Errorf("failed to create agent: %v", err)
 	}
 	defer func() { _ = mcpAgent.Close() }()
+	// TODO(TAS-18): once app.App is created here, add: defer appInstance.Close()
+	// app.Close() stops background goroutines and waits for in-flight steps to
+	// finish before returning — satisfying TAS-14's graceful-quit requirement.
 
 	// Initialize hook executor if hooks are configured
 	// Get model name for display

internal/ui/model.go

@@ -265,6 +265,11 @@ func (m *AppModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 
 	// ── Input submitted ──────────────────────────────────────────────────────
 	case submitMsg:
+		// Handle /quit (and its aliases) before sending to the app layer:
+		// look up the command and check if it resolves to "/quit".
+		if cmd := GetCommandByName(msg.Text); cmd != nil && cmd.Name == "/quit" {
+			return m, tea.Quit
+		}
 		if m.appCtrl != nil {
 			// app.Run() handles queueing internally if a step is in progress.
 			m.appCtrl.Run(msg.Text)