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/scripts/brand-header

@@ -0,0 +1,36 @@
+#!/bin/sh
+# motd-assist :: brand-header — Rahmen-Header mit Hostname und Betriebssystem
+set -u
+
+E=$(printf '\033')
+BLUE="${E}[1;38;5;75m"
+ORANGE="${E}[1;38;5;214m"
+DIM="${E}[2m"
+BOLD="${E}[1m"
+R="${E}[0m"
+
+HOST=$(hostname 2>/dev/null || echo localhost)
+HOST=${HOST%%.*}
+
+OS=Linux
+if [ -r /etc/os-release ]; then
+	. /etc/os-release
+	OS=${PRETTY_NAME:-Linux}
+fi
+
+W=${COLUMNS:-72}
+case $W in ''|*[!0-9]*) W=72 ;; esac
+[ "$W" -gt 78 ] && W=78
+[ "$W" -lt 44 ] && W=44
+
+INNER=$((W - 2))
+PLAIN=" ${HOST} - ${OS} "
+PAD=$((INNER - ${#PLAIN}))
+[ "$PAD" -lt 0 ] && PAD=0
+
+RULE=$(awk -v n="$INNER" 'BEGIN{s="";for(i=0;i<n;i++)s=s "─";printf "%s",s}')
+SPACES=$(printf '%*s' "$PAD" '')
+
+printf '%b\n' "$BLUE╭$R$DIM$RULE$R$BLUE╮$R"
+printf '%b\n' "$BLUE│$R $BOLD$BLUE$HOST$R $DIM·$R $ORANGE$OS$R$SPACES $BLUE│$R"
+printf '%b\n' "$BLUE╰$R$DIM$RULE$R$BLUE╯$R"