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/preset/preset_test.go

@@ -0,0 +1,59 @@
+package preset
+
+import (
+	"testing"
+)
+
+func TestBuiltinPresetsLoad(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+
+	presets, err := LoadAll()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(presets) < 3 {
+		t.Errorf("Built-in-Presets fehlen: %d geladen", len(presets))
+	}
+	for _, p := range presets {
+		if p.Source != "builtin" {
+			t.Errorf("Preset %s: Source = %s", p.Name, p.Source)
+		}
+		if len(p.Scripts) == 0 {
+			t.Errorf("Preset %s hat keine Scripte", p.Name)
+		}
+	}
+}
+
+func TestSaveAndUserOverride(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+
+	p := Preset{
+		Name:        "minimal",
+		Description: "eigene Variante",
+		Scripts:     []Entry{{Name: "00-header", Enabled: true}},
+	}
+	if _, err := SaveUser(p); err != nil {
+		t.Fatal(err)
+	}
+
+	presets, err := LoadAll()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, got := range presets {
+		if got.Name == "minimal" {
+			if got.Source != "user" || got.Description != "eigene Variante" {
+				t.Errorf("User-Override greift nicht: %+v", got)
+			}
+			return
+		}
+	}
+	t.Fatal("minimal nicht gefunden")
+}
+
+func TestDeleteUserRejectsUnknown(t *testing.T) {
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+	if err := DeleteUser("gibt-es-nicht"); err == nil {
+		t.Error("Löschen eines fehlenden Presets sollte fehlschlagen")
+	}
+}