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/assets/presets/server-dashboard.json

@@ -0,0 +1,13 @@
+{
+  "name": "server-dashboard",
+  "description": "Server-Übersicht: Branding-Header, Sysinfo-Balken und Update-Hinweis",
+  "scripts": [
+    { "name": "00-header", "enabled": false },
+    { "name": "10-help-text", "enabled": false },
+    { "name": "50-motd-news", "enabled": false },
+    { "name": "10-brand-header", "enabled": true, "install": "brand-header" },
+    { "name": "20-sysinfo-panel", "enabled": true, "install": "sysinfo-panel" },
+    { "name": "90-updates-available", "enabled": true },
+    { "name": "95-footer", "enabled": true, "install": "footer" }
+  ]
+}