feat(flake): expose lib.mkSystem for external consumers

Surface mkSystem as a flake output and make its host resolution
non-throwing: internal hosts (this repo's hosts/<name>/) are still
imported automatically, but a consumer using OmniXY as a flake input
can now pass their own host via `modules` instead. First brick of the
dependency-model framework; the core module aggregate (nixosModules)
follows so a consumer's host can import OmniXY core.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/mksystem.nix

@@ -70,13 +70,18 @@ let
   # fall back to a flat `hosts/<name>.nix`.
   hostDir = ../hosts/${name};
   hostFile = ../hosts + "/${name}.nix";
-  hostConfig =
+  # Internal hosts (this repo's hosts/<name>/) are imported automatically.
+  # External consumers using this flake as an input have no host dir here,
+  # so they pass their host config through `modules` and we add nothing.
+  hostConfigModules =
     if builtins.pathExists hostDir then
-      hostDir
+      [ hostDir ]
     else if builtins.pathExists hostFile then
-      hostFile
+      [ hostFile ]
+    else if modules != [ ] then
+      [ ]
     else
-      throw "mksystem: no host config for '${name}' (looked at ${toString hostDir} and ${toString hostFile})";
+      throw "mksystem: no host config for '${name}' — add hosts/${name}/ or pass one via `modules` (looked at ${toString hostDir} and ${toString hostFile})";
 
   userNixosConfig = ../users/${user}/nixos.nix;
   userHMConfig = ../users/${user}/home-manager.nix;
@@ -101,10 +106,10 @@ nixpkgs.lib.nixosSystem {
     { nixpkgs.hostPlatform = system; }
     { nixpkgs.overlays = overlays; }
     { nixpkgs.config.allowUnfree = true; }
-
-    # Per-host configuration.
-    hostConfig
   ]
+  # Per-host configuration: this repo's hosts/<name>/ when present;
+  # external consumers supply their host through `modules`.
+  ++ hostConfigModules
   # Per-user system-level config (account, shell, sudo) — only if the
   # user file exists. Skipping silently is fine; some hosts share users
   # defined elsewhere (e.g. omni's users.nix).