feat(nix): bootstrap clan-core for sunken-ship + phantom-ship 🏴‍☠️

Stage 4a of the dendritic + clan migration. Both servers now live under
clan.machines (via nixos/flake-modules/clan.nix) and clan-core generates
their nixosConfigurations for us; the previous per-host flake-modules
are removed.

Notes:
- clan.core.enableRecommendedDefaults = false on both machines so we
  keep the existing dhcpcd / non-networkd / non-resolved stack. Services
  like dnsmasq, navidrome, and the existing wireless setup break with
  the clan defaults on.
- dotfiles-rebuild timer is untouched (safety net). Replacing it with
  clan machines update / dm-pull-deploy comes in 4e.
- mac stays outside the clan as admin only.

Verified: `clan machines list --flake path:…/nixos` returns both hosts;
both servers rebuild cleanly and all services (navidrome, cloudflare-
tunnel, fitness-bot, dnsmasq, openclaw-gateway, sshd) stay active.

nixos/flake.nix

@@ -24,6 +24,10 @@
 
     nix-openclaw.url = "github:openclaw/nix-openclaw";
     nix-openclaw.inputs.nixpkgs.follows = "nixpkgs";
+
+    clan-core.url = "https://git.clan.lol/clan/clan-core/archive/main.tar.gz";
+    clan-core.inputs.nixpkgs.follows = "nixpkgs";
+    clan-core.inputs.flake-parts.follows = "flake-parts";
   };
 
   outputs = inputs @ { flake-parts, import-tree, ... }: