feat(node): package Ubuntu desktop setup and trusted access

apps/node-agent/packaging/postinst

@@ -0,0 +1,34 @@
+#!/bin/sh
+set -eu
+case "$1" in
+  configure)
+    if ! getent passwd mission-core-node >/dev/null; then
+      adduser --system --group --home /var/lib/mission-core-node --no-create-home --disabled-login mission-core-node
+    fi
+    mc_node_ssh_snippet=/etc/ssh/sshd_config.d/60-mission-core-node.conf
+    mc_node_ssh_template=/usr/share/mission-core-node/60-mission-core-node.conf
+    if [ -L "$mc_node_ssh_snippet" ] || { [ -e "$mc_node_ssh_snippet" ] && ! cmp -s "$mc_node_ssh_template" "$mc_node_ssh_snippet"; }; then
+      echo "Mission Core Node: existing custom SSH snippet preserved; configuration conflict." >&2
+      exit 1
+    fi
+    install -D -m 0644 "$mc_node_ssh_template" "$mc_node_ssh_snippet"
+    if [ -d /run/systemd/system ]; then
+      install -d -m 0755 /run/sshd
+      /usr/sbin/sshd -t
+      mc_node_ssh_config=$(/usr/sbin/sshd -T)
+      if ! printf '%s\n' "$mc_node_ssh_config" | grep -Fx 'authorizedkeyscommand /usr/lib/mission-core-node/node-agent ssh-keys %u' >/dev/null; then
+        echo "Mission Core Node: another AuthorizedKeysCommand overrides Node SSH access. Existing configuration was preserved; resolve this conflict before accepting setup." >&2
+        exit 1
+      fi
+      if ! printf '%s\n' "$mc_node_ssh_config" | grep -Fx 'authorizedkeyscommanduser mission-core-node' >/dev/null; then
+        echo "Mission Core Node: conflicting AuthorizedKeysCommandUser; existing configuration was preserved." >&2
+        exit 1
+      fi
+      systemctl daemon-reload
+      systemctl enable --now ssh.service
+      systemctl try-reload-or-restart ssh.service
+      systemctl enable mission-core-node.service
+      systemctl restart mission-core-node.service
+    fi
+    ;;
+esac