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/footer

@@ -0,0 +1,14 @@
+#!/bin/sh
+# motd-assist :: footer — dezente Kernel-/Uptime-Zeile
+set -u
+
+E=$(printf '\033')
+DIM="${E}[2m"
+R="${E}[0m"
+
+K=$(uname -r 2>/dev/null || echo unknown)
+UP=$(uptime -p 2>/dev/null)
+[ -n "$UP" ] || UP=$(awk '{d=int($1/86400); h=int(($1%86400)/3600); printf "up %d days %d hours", d, h}' /proc/uptime 2>/dev/null)
+
+[ -n "$UP" ] && printf '%b\n' "${DIM}${K} · ${UP}${R}"
+exit 0