Keep cobra's output streams at their defaults in cli.Main

cli.Main no longer calls SetOut or SetErr. SetOut also moved the usage
and an unknown help topic, which cobra writes to stderr, onto stdout.
execute now takes the command tree, so the tests build it, set its
arguments and output streams, and run it; the exit line goes to the
command's error stream, which outside a test is stderr as before.

Model: opus-5-5

internal/cli/cli_test.go

@@ -14,7 +14,12 @@ func TestVersionPrintsVersion(t *testing.T) {
 
 	var stdout, stderr bytes.Buffer
 
-	status := cli.Execute("1.2.3", []string{"version"}, &stdout, &stderr)
+	root := cli.RootCmd("1.2.3")
+	root.SetArgs([]string{"version"})
+	root.SetOut(&stdout)
+	root.SetErr(&stderr)
+
+	status := cli.Execute(root)
 	if status != 0 {
 		t.Fatalf("status = %d, want 0; stderr: %q", status, stderr.String())
 	}
@@ -30,7 +35,12 @@ func TestUnknownCommandFails(t *testing.T) {
 
 	var stdout, stderr bytes.Buffer
 
-	status := cli.Execute("1.2.3", []string{"frobnicate"}, &stdout, &stderr)
+	root := cli.RootCmd("1.2.3")
+	root.SetArgs([]string{"frobnicate"})
+	root.SetOut(&stdout)
+	root.SetErr(&stderr)
+
+	status := cli.Execute(root)
 	if status == 0 {
 		t.Fatal("status = 0, want non-zero")
 	}