Initialize narratio Go module and CLI scaffold

internal/app/plan.go

@@ -0,0 +1,11 @@
+package app
+
+import (
+	"context"
+	"io"
+)
+
+// Plan is a placeholder for future stage planning behavior.
+func Plan(_ context.Context, _ []string, out io.Writer) error {
+	return placeholder(out, "plan")
+}