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>

flake.nix

@@ -218,6 +218,19 @@
       };
     in
     {
+      # Reusable library for external consumers. Use this flake as an
+      # input and build a host with the Omnixient conventions from your own
+      # repo, without forking:
+      #
+      #   omni.lib.mkSystem "myhost" {
+      #     settings = { ... };          # your identity (see settings.nix)
+      #     modules  = [ ./hosts/myhost ];
+      #   };
+      #
+      # mkSystem bakes in Omnixient's nixpkgs, overlays, and inputs
+      # (home-manager, sops-nix, …), so you don't redeclare them.
+      lib.mkSystem = mkSystem;
+
       # NixOS configurations.
       #
       # This public template ships the `example` host (copy-me starter) and