feat(secrets): add push-host-keys.sh; integrate into sync/recover scripts

New script: scripts/secrets/push-host-keys.sh
- Pushes newly-generated SSH host keys from host-keys/ to already-running
  NixOS hosts after sync-host-keys.sh --regenerate-all-keys.
- Before pushing any key, checks that .sops.yaml and secrets/*.yaml are
  committed and pushed to the remote Gitea flake (hosts rebuild from there,
  so recipient changes must land first); offers to auto-commit/push if not.
- Reads /etc/flake-target from each host to confirm which key to install,
  handling the case where multiple flake targets share a hostname.
- Deduplicates by hostname in --all mode; skips hand-registered targets
  that have no host-keys/ entry.
- --dry-run, --skip-git-check, SSH_USER override (default: nixos).

sync-host-keys.sh --regenerate-all-keys:
- Updated pre-confirmation warning to distinguish already-running hosts
  (need push-host-keys.sh) from not-yet-deployed hosts (need installer
  image rebuild).
- Added next-steps block after regeneration completes pointing to
  push-host-keys.sh --all.

recover-hosts.sh:
- Header and SSH host key mismatch warn now cross-reference
  push-host-keys.sh as the proactive (pre-drift) alternative.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/recover-hosts.sh

@@ -12,6 +12,9 @@
 # Fixes applied automatically (then prompts before rebuilding):
 #   1. SSH host key drift — live key no longer matches host-keys/<target>_ssh_host_ed25519_key
 #      Fix: scp the registered key back and restore it (needs sudo once per host).
+#      To push new keys proactively (before drift, e.g. right after
+#      sync-host-keys.sh --regenerate-all-keys), use instead:
+#        scripts/secrets/push-host-keys.sh --all
 #   2. Stale/invalid GitHub access token — the rendered nix-github-token.conf has
 #      a token GitHub rejects (401), blocking any rebuild that fetches disko or
 #      other public GitHub flake inputs.
@@ -131,7 +134,7 @@ for host in "${HOSTNAMES[@]}"; do
     if [ "$live" = "$want" ]; then
       info "SSH host key OK"
     else
-      warn "SSH host key MISMATCH (live ≠ host-keys/)"
+      warn "SSH host key MISMATCH (live ≠ host-keys/) -- use push-host-keys.sh proactively next time"
       echo "       live:       $live"
       echo "       registered: $want"
       host_broken=true