sysmig: migration runner + initial 4 migrations (swap, daemon trims)

- up/down/status interface, history in /var/lib/sysmig/applied
- 001 swap 2GB + swappiness=10, 002-004 trim unneeded daemons (~56MB)
- google-osconfig-agent intentionally kept
- all messages/comments in ASCII English (console font safety)

migrations/003-purge-exim4.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# 003-purge-exim4 - Remove default local mail server (~21MB)
+# Unnecessary if local mail (e.g. cron output) is not used.
+# Down reinstalls with Debian defaults (local delivery only).
+set -euo pipefail
+
+export DEBIAN_FRONTEND=noninteractive
+
+case "${1:-}" in
+  up)   systemctl disable --now exim4.service 2>/dev/null || true
+        apt-get purge -y exim4-daemon-light >/dev/null
+        apt-get autoremove --purge -y >/dev/null
+        echo "    exim4 stopped + package purged + deps cleaned" ;;
+  down) apt-get update -qq
+        apt-get install -y exim4-daemon-light >/dev/null
+        systemctl enable --now exim4.service
+        echo "    exim4 reinstalled + restarted (default config)" ;;
+esac