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/001-create-swap.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# 001-create-swap - Create 2GB swap + fstab entry + vm.swappiness=10
+# up:   enable swap (skipped if swap already active)
+# down: disable swap and remove file/fstab/sysctl entries
+set -euo pipefail
+
+case "${1:-}" in
+  up)
+    if swapon --show --noheadings | grep -q .; then
+      echo "    swap already active - skipping"
+      exit 0
+    fi
+    if [[ ! -f /swapfile ]]; then
+      fallocate -l 2G /swapfile || {
+        dd if=/dev/zero of=/swapfile bs=1M count=2048 status=none
+      }
+      chmod 600 /swapfile
+      mkswap /swapfile >/dev/null
+    fi
+    swapon /swapfile
+    grep -q '^/swapfile' /etc/fstab || echo '/swapfile none swap sw 0 0' >> /etc/fstab
+    if ! grep -rqs 'vm.swappiness' /etc/sysctl.d/ /etc/sysctl.conf; then
+      echo 'vm.swappiness=10' > /etc/sysctl.d/99-swap.conf
+      sysctl -p /etc/sysctl.d/99-swap.conf >/dev/null
+    fi
+    echo "    swap 2GB enabled + fstab entry + vm.swappiness=10"
+    ;;
+  down)
+    swapoff /swapfile
+    rm -f /swapfile
+    sed -i '\|^/swapfile|d' /etc/fstab
+    grep -q '^vm.swappiness=10$' /etc/sysctl.d/99-swap.conf 2>/dev/null \
+      && rm -f /etc/sysctl.d/99-swap.conf || true
+    echo "    swap removed"
+    ;;
+esac

migrations/002-disable-networkd-dispatcher.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# 002-disable-networkd-dispatcher - Stop network event hook daemon (~27MB)
+# Unnecessary on a simple server with static network config. Package kept.
+set -euo pipefail
+
+case "${1:-}" in
+  up)   systemctl disable --now networkd-dispatcher.service
+        echo "    networkd-dispatcher stopped and disabled" ;;
+  down) systemctl enable --now networkd-dispatcher.service
+        echo "    networkd-dispatcher re-enabled" ;;
+esac

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

migrations/004-purge-haveged.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# 004-purge-haveged - Remove entropy daemon (~8MB)
+# Kernel 6.12 collects enough entropy on its own; haveged is redundant.
+set -euo pipefail
+
+export DEBIAN_FRONTEND=noninteractive
+
+case "${1:-}" in
+  up)   systemctl disable --now haveged.service 2>/dev/null || true
+        apt-get purge -y haveged >/dev/null
+        apt-get autoremove --purge -y >/dev/null
+        echo "    haveged stopped + package purged" ;;
+  down) apt-get update -qq
+        apt-get install -y haveged >/dev/null
+        systemctl enable --now haveged.service
+        echo "    haveged reinstalled + restarted" ;;
+esac