feat(provision): Phase 2 — migrate SSH host keys to clan vars

Replaces the gitignored host-keys/ directory with clan vars as the
authoritative storage for SSH host keys. Keys are now generated as
sops-binary-encrypted clan var files (admin-key only) and checked into
vars/per-machine/<target>/openssh/, eliminating the plaintext private
key that previously had to live outside the repo.

Changes:
- modules/clan/ssh-host-key.nix: clan vars generator for the ed25519
  SSH host key pair (neededFor="activation" — not mapped to sops.secrets,
  delivered via tarball baking for LXC or --pre-format-files for VMs)
- flake.nix: add clanCore module + required settings to every mkTarget;
  deduplicate bundled disko/sops-nix via follows; all 27 hosts eval clean
- flake.lock: updated to reflect the new follows constraints
- scripts/lib/clan-vars.sh: new helper library with
  clan_ssh_key_exists / clan_ssh_pubkey_path / clan_decrypt_ssh_key /
  clan_generate_ssh_key for use by the provisioning and sync scripts
- scripts/secrets/sync-host-keys.sh: queue_host_sync() now checks clan
  vars first; generates via clan_generate_ssh_key if no key exists;
  derives age fingerprint from clan pub key for .sops.yaml registration
- scripts/proxmox/create-proxmox-resource.sh: key management simplified
  (sync-host-keys.sh now generates the key if missing, so the inline
  prepare-host-key.sh call is gone); sync_remote_host_keys() decrypts
  the clan key into a temp dir and scps just the two files to the node
  when a clan key exists, falling back to the old host-keys/ scp for
  any remaining legacy entries

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B2EJ4qTsM5KUqhS5c3GAwx

scripts/secrets/sync-host-keys.sh

@@ -39,6 +39,8 @@ source "${repo_root}/scripts/lib/ssh-host-keys.sh"
 source "${repo_root}/scripts/lib/sops-age.sh"
 # shellcheck source=../lib/confirm.sh
 source "${repo_root}/scripts/lib/confirm.sh"
+# shellcheck source=../lib/clan-vars.sh
+source "${repo_root}/scripts/lib/clan-vars.sh"
 
 mkdir -p "$keydir"
 
@@ -146,13 +148,15 @@ dry_run=0
 queue_host_sync() {
   local host="$1"
   local keyfile="${keydir}/${host}_ssh_host_ed25519_key"
-  local has_local_key=0 has_anchor=0
+  local has_local_key=0 has_clan_key=0 has_anchor=0
   [[ -f "$keyfile" ]] && has_local_key=1
+  clan_ssh_key_exists "$host" "$repo_root" && has_clan_key=1
   grep -qE "^  - &${host} age1" "$sops_yaml" && has_anchor=1
 
-  if [[ "$has_local_key" -eq 0 && "$has_anchor" -eq 1 ]]; then
+  if [[ "$has_local_key" -eq 0 && "$has_clan_key" -eq 0 && "$has_anchor" -eq 1 ]]; then
     echo "SKIP ${host}: .sops.yaml already has an &${host} anchor, but"
-    echo "     host-keys/${host}_ssh_host_ed25519_key is missing locally."
+    echo "     neither host-keys/${host}_ssh_host_ed25519_key nor"
+    echo "     vars/per-machine/${host}/openssh/ exist locally."
     echo "     Not generating a replacement -- it wouldn't match whatever's"
     echo "     already registered (and possibly deployed). Remove the"
     echo "     &${host} line from .sops.yaml first if you really want a"
@@ -160,21 +164,26 @@ queue_host_sync() {
     return 1
   fi
 
-  if [[ "$has_local_key" -eq 0 ]]; then
+  if [[ "$has_local_key" -eq 0 && "$has_clan_key" -eq 0 ]]; then
     if [[ "$dry_run" -eq 1 ]]; then
-      echo "[dry-run] ${host}: would generate host key"
+      echo "[dry-run] ${host}: would generate host key via clan vars"
     else
-      echo "==> ${host}: generating host key"
-      generate_host_ed25519_key "$host" "$keyfile"
+      echo "==> ${host}: generating host key via clan vars"
+      clan_generate_ssh_key "$host" "$repo_root"
+      has_clan_key=1
     fi
+  elif [[ "$has_clan_key" -eq 1 ]]; then
+    echo "==> ${host}: clan-managed SSH host key already present"
   else
-    echo "==> ${host}: host key already present"
+    echo "==> ${host}: host key already present (host-keys/)"
   fi
 
   if [[ "$has_anchor" -eq 0 ]]; then
     local age_pub
     if [[ "$dry_run" -eq 1 ]]; then
       age_pub="dry-run-placeholder-not-a-real-key"
+    elif [[ "$has_clan_key" -eq 1 ]]; then
+      age_pub="$(ssh_pubkey_to_age "$(clan_ssh_pubkey_path "$host" "$repo_root")")"
     else
       age_pub="$(ssh_pubkey_to_age "${keyfile}.pub")"
     fi