nix: export the CI server as nixosModules.cc-ci-server

The whole server (every service module, the harness tooling, sops wiring,
acme-dns) becomes one reusable module, nix/modules/default.nix, so another
flake can run cc-ci on a host it defines. First consumer: the
cc-ci-orchestrator repo's `#cc-ci` host, which runs the CI server and the
orchestrator together on one Hetzner machine.

Two things the modules hard-coded become options (nix/modules/options.nix):
- cc-ci.publicIPv4 — acme-dns's listen address and ns-acme glue record.
- cc-ci.sopsFile — the secrets.yaml path; defaults to the secrets/ submodule,
  but a consumer that imports cc-ci as a plain input (no private submodule)
  points it at the deployed --recursive checkout and sops-nix reads it at
  activation (validateSopsFiles off for that case).

The standalone host (nix/hosts/cc-ci-hetzner) now only carries hardware,
networking and identity and imports the module via the flake. Verified: the
`#cc-ci` system derivation is byte-identical before and after
(/nix/store/ckp1244bz86fz3qbx81n5kx60c1lak3m-…531670d.drv on both).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FqkQq3CDmFWcQ7u1LzoyRz

nix/modules/acme-dns.nix

@@ -3,18 +3,19 @@
 # This host is authoritative only for acme.commoninternet.net. Gandi continues
 # to own commoninternet.net; it delegates this narrow zone and one permanent
 # _acme-challenge CNAME manually. No Gandi credential is present here.
-{ pkgs, ... }:
+{ config, pkgs, ... }:
 let
+  publicIPv4 = config.cc-ci.publicIPv4;
   acmeDnsConfig = pkgs.writeText "cc-ci-acme-dns.conf" ''
     [general]
-    listen = "91.98.47.73:53"
+    listen = "${publicIPv4}:53"
     protocol = "both4"
     domain = "acme.commoninternet.net"
     nsname = "ns-acme.commoninternet.net"
     nsadmin = "hostmaster.commoninternet.net"
     records = [
       "acme.commoninternet.net. NS ns-acme.commoninternet.net.",
-      "ns-acme.commoninternet.net. A 91.98.47.73",
+      "ns-acme.commoninternet.net. A ${publicIPv4}",
     ]
     debug = false
 
@@ -48,6 +49,8 @@ let
   '';
 in
 {
+  imports = [ ./options.nix ];
+
   users.groups.acme-dns = { };
   users.users.acme-dns = {
     isSystemUser = true;