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/ubuntu-default.json

@@ -0,0 +1,16 @@
+{
+  "name": "ubuntu-default",
+  "description": "Ubuntu-Standard: alle bekannten System-Scripte aktiviert",
+  "scripts": [
+    { "name": "00-header", "enabled": true },
+    { "name": "10-help-text", "enabled": true },
+    { "name": "50-landscape-sysinfo", "enabled": true },
+    { "name": "50-motd-news", "enabled": true },
+    { "name": "80-livepatch", "enabled": true },
+    { "name": "90-updates-available", "enabled": true },
+    { "name": "91-release-upgrade", "enabled": true },
+    { "name": "95-hwe-eol", "enabled": true },
+    { "name": "97-overlayroot", "enabled": true },
+    { "name": "98-fsck-at-reboot", "enabled": true }
+  ]
+}