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/root_unix.go

@@ -0,0 +1,8 @@
+//go:build unix
+
+package motd
+
+import "os"
+
+// IsRoot meldet, ob der Prozess mit EUID 0 läuft.
+func IsRoot() bool { return os.Geteuid() == 0 }