add tui to no-channel build variant

cmd/cmd_tui_stub.go

@@ -0,0 +1,12 @@
+//go:build !with_tui
+
+package main
+
+import "fmt"
+
+var tuiEnabled = false
+
+func tuiCmd() {
+	fmt.Println("TUI is not included in this build.")
+	fmt.Println("Install the no-channel variant to use `clawgo tui`.")
+}