fix(lxc): prevent SSH host key deletion on every rebuild; add recovery script

NixOS's etc activation removes files that were in a previous generation's
environment.etc but absent from the current one -- even real copies, not
only symlinks.  LXC tarballs bake the host key into environment.etc (via
NIXOS_HOST_KEYS_DIR), but every subsequent nixos-rebuild switch lacks that
env var, so the key is removed as "obsolete".  sops-nix derives its age
decryption key from /etc/ssh/ssh_host_ed25519_key, so deletion cascades
into "Error getting data key: 0 successful groups required, got 0" for
every sops secret on the host.

Fix: two activation scripts bracket the etc step.
  preserveSshHostKey (no deps, runs before etc): copies the live key to
    /run (tmpfs) before etc can delete it.
  restoreSshHostKey (deps=[etc], runs after etc): reinstalls via `install`
    if etc removed the key.  The resulting file is not tracked in either
    generation's environment.etc, so subsequent rebuilds leave it alone.

scripts/recover-hosts.sh: restore both private and public key files (not
just the private key), use install(1) for atomic mode setting, and add a
post-rebuild sops-nix verification step to confirm success.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014zT1L6hmsq6i1evAEH7dmi

modules/platforms/lxc.nix

@@ -106,6 +106,40 @@ in
     };
   };
 
+  # NixOS's etc activation removes any /etc file that was in the previous
+  # generation's environment.etc but is absent from the current one — even
+  # real (non-symlink) copies.  On every routine nixos-rebuild switch/test that
+  # lacks NIXOS_HOST_KEYS_DIR the key is absent from environment.etc, so it
+  # gets removed as "obsolete".  sops-nix derives its age decryption key from
+  # /etc/ssh/ssh_host_ed25519_key; deletion cascades into every sops secret
+  # failing with "Error getting data key: 0 successful groups required, got 0".
+  #
+  # Fix: two activation scripts that bracket the etc step.
+  #   preserveSshHostKey — no deps, runs before etc — saves the live key to
+  #                        /run (tmpfs) before etc can delete it.
+  #   restoreSshHostKey  — deps=[etc], runs after etc — reinstalls the key via
+  #                        `install` (atomic, sets mode) if etc removed it.
+  #                        The resulting file is not registered in environment.etc
+  #                        for either the previous or current generation, so
+  #                        subsequent rebuilds leave it alone permanently.
+  system.activationScripts.preserveSshHostKey = ''
+    if [ -f /etc/ssh/ssh_host_ed25519_key ]; then
+      cp /etc/ssh/ssh_host_ed25519_key /run/sshd-host-key-preserve.tmp
+      cp /etc/ssh/ssh_host_ed25519_key.pub /run/sshd-host-key-preserve.pub.tmp
+    fi
+  '';
+
+  system.activationScripts.restoreSshHostKey = {
+    deps = [ "etc" ];
+    text = ''
+      if [ ! -f /etc/ssh/ssh_host_ed25519_key ] && [ -f /run/sshd-host-key-preserve.tmp ]; then
+        install -m 0600 /run/sshd-host-key-preserve.tmp /etc/ssh/ssh_host_ed25519_key
+        install -m 0644 /run/sshd-host-key-preserve.pub.tmp /etc/ssh/ssh_host_ed25519_key.pub
+      fi
+      rm -f /run/sshd-host-key-preserve.tmp /run/sshd-host-key-preserve.pub.tmp
+    '';
+  };
+
   # virtualisation/proxmox-lxc.nix (imported above) registers the Nix
   # store DB via a systemd service (register-nix-paths) -- it never runs
   # an activation script at all. Confirmed live this means neither