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/ui/commands.go

@@ -0,0 +1,110 @@
+package ui
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+
+	tea "github.com/charmbracelet/bubbletea"
+
+	"motd-assist/internal/motd"
+	"motd-assist/internal/preset"
+)
+
+type discoverMsg struct {
+	scripts []motd.Script
+	host    motd.HostInfo
+	err     error
+}
+
+type presetsMsg struct {
+	presets []preset.Preset
+	err     error
+}
+
+type previewMsg struct {
+	version int
+	text    string
+}
+
+type applyDoneMsg struct{ err error }
+
+func discoverCmd(root string) tea.Cmd {
+	return func() tea.Msg {
+		scripts, err := motd.Discover(root)
+		return discoverMsg{scripts: scripts, host: motd.DetectHost(root), err: err}
+	}
+}
+
+func presetsCmd() tea.Cmd {
+	return func() tea.Msg {
+		presets, err := preset.LoadAll()
+		return presetsMsg{presets: presets, err: err}
+	}
+}
+
+// previewCmd rendert die Arbeitskopie als Live-Vorschau (asynchron, damit
+// langsame Scripte die TUI nicht blockieren).
+func (m *Model) previewCmd() tea.Cmd {
+	m.previewBusy = true
+	m.previewVersion++
+	version := m.previewVersion
+	width := m.previewWidth()
+	scripts := make([]motd.PreviewScript, 0, len(m.working))
+	for _, r := range m.working {
+		if r.Remove {
+			continue
+		}
+		ps := motd.PreviewScript{Name: r.Name, Enabled: r.Enabled}
+		if r.Exists {
+			ps.Path = filepath.Join(motd.MotdDir(m.root), r.Name)
+		} else {
+			ps.Source = r.Install
+		}
+		scripts = append(scripts, ps)
+	}
+	return func() tea.Msg {
+		return previewMsg{version: version, text: motd.Render(scripts, width)}
+	}
+}
+
+// previewWidth ist die nutzbare Breite innerhalb der Preview-Umrandung.
+func (m *Model) previewWidth() int {
+	w := m.width - 4 // Rahmen (2) + Padding (2)
+	if w < 40 {
+		w = 40
+	}
+	if w > 100 {
+		w = 100
+	}
+	return w
+}
+
+// applyExec handelt das Terminal an sudo weiter (interaktive
+// Passwortabfrage) und startet danach die TUI weiter.
+func applyExec(plan motd.Plan) tea.Cmd {
+	planFile, err := plan.WriteFile()
+	if err != nil {
+		return func() tea.Msg { return applyDoneMsg{err: err} }
+	}
+	self, err := os.Executable()
+	if err != nil {
+		os.Remove(planFile)
+		return func() tea.Msg { return applyDoneMsg{err: err} }
+	}
+	var cmd *exec.Cmd
+	if motd.IsRoot() {
+		cmd = exec.Command(self, "apply-state", planFile)
+	} else {
+		sudo, err := exec.LookPath("sudo")
+		if err != nil {
+			os.Remove(planFile)
+			return func() tea.Msg { return applyDoneMsg{err: err} }
+		}
+		cmd = exec.Command(sudo, "-p", "[motd-assist] Passwort für %p: ", self, "apply-state", planFile)
+	}
+	return tea.ExecProcess(cmd, func(err error) tea.Msg {
+		os.Remove(planFile)
+		return applyDoneMsg{err: err}
+	})
+}