Record the Debian 13 upgrade and turn off the VSOCK SSH generator

The server now runs Debian 13.7. systemd 257's ssh generator logs a
harmless AF_VSOCK error on netcup VMs, so the common role masks it.
The plan file now records how the upgrade went and what's left.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

roles/common/tasks/main.yml

@@ -41,3 +41,13 @@
       X11Forwarding no
     validate: /usr/sbin/sshd -t -f %s
   notify: reload ssh
+
+# systemd 257 (Debian 13) adds a generator that offers SSH over VSOCK. netcup
+# VMs have no VSOCK device, so it only logs "Failed to query local AF_VSOCK
+# CID" to the console. Normal SSH doesn't use it.
+- name: Turn off systemd's SSH-over-VSOCK generator
+  ansible.builtin.file:
+    src: /dev/null
+    dest: /etc/systemd/system-generators/systemd-ssh-generator
+    state: link
+    force: true