feat: add motd-assist TUI for managing dynamic MOTDs

Initial Go project built with Bubble Tea and Lip Gloss:
- Live preview and management of /etc/update-motd.d scripts
- Embedded presets (ubuntu-default, minimal, server-dashboard)
- Bundled POSIX sh style scripts (brand header, sysinfo panel, footer)
- Snapshot/restore safety around applying changes
- CLI subcommands preview, apply, restore, list-presets
- Unit tests for detect, sanitize, and apply logic

internal/motd/discover_test.go

@@ -0,0 +1,40 @@
+package motd
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+)
+
+func TestDiscoverListsValidNamesSorted(t *testing.T) {
+	root := t.TempDir()
+	dir := MotdDir(root)
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	for _, name := range []string{"10-sysinfo", "00-header", "notes.txt", ".hidden", "README"} {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte("#!/bin/sh\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	scripts, err := Discover(root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var names []string
+	for _, s := range scripts {
+		names = append(names, s.Name)
+	}
+	// README ist ein valider run-parts-Name und bleibt dabei.
+	if !reflect.DeepEqual(names, []string{"00-header", "10-sysinfo", "README"}) {
+		t.Errorf("names = %v", names)
+	}
+}
+
+func TestDiscoverMissingDir(t *testing.T) {
+	if _, err := Discover(t.TempDir()); err == nil {
+		t.Error("Fehler erwartet, wenn update-motd.d fehlt")
+	}
+}