Extract the ssh debug commands into a transport neutral commands package (#1958)

Migrate commands from ssh into a transport neutral package

 - add golden tests validating that output remains unchanged
 - move commands to commands.go

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

test/helpers.go

@@ -0,0 +1,29 @@
+package test
+
+import (
+	"flag"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Update is the -update flag: Golden rewrites its file instead of comparing against it.
+var Update = flag.Bool("update", false, "rewrite golden files instead of comparing against them")
+
+// Golden compares got with the file at path, or rewrites the file when -update is set.
+func Golden(t *testing.T, path, got string) {
+	t.Helper()
+
+	if *Update {
+		require.NoError(t, os.MkdirAll(filepath.Dir(path), 0755))
+		require.NoError(t, os.WriteFile(path, []byte(got), 0644))
+		return
+	}
+
+	want, err := os.ReadFile(path)
+	require.NoError(t, err, "missing golden file, run with -update")
+	assert.Equal(t, string(want), got)
+}