feat(flake): add consumer template (nix flake init -t)

Expose templates.default — a starter flake that consumes OmniXY as an
input (dependency model): omnixy.lib.mkSystem + nixosModules.omnixy +
a placeholder host. Lets adopters track OmniXY with `nix flake update`
instead of forking. See templates/default/README.md.

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

templates/default/flake.nix

@@ -0,0 +1,41 @@
+{
+  description = "My NixOS machine(s), built on Omnixient";
+
+  inputs = {
+    # Pin Omnixient. Point this at the canonical repo or your own fork, and
+    # ideally pin a release tag (…/omni/v1.0.0) for reproducible updates:
+    #   nix flake update omni   # pull the latest Omnixient, never touches your config
+    omni.url = "github:<your-org>/omnixient";
+    # Reuse Omnixient's nixpkgs so you evaluate a single package set.
+    nixpkgs.follows = "omni/nixpkgs";
+  };
+
+  outputs =
+    { omni, ... }:
+    let
+      # Your identity + machine settings. Omnixient core reads `settings.theme`;
+      # the home config reads the rest. Edit these.
+      settings = {
+        user = "alice";
+        gitName = "Alice Example";
+        gitEmail = "alice@example.com";
+        hostName = "myhost";
+        timeZone = "America/New_York";
+        theme = "tokyo-night"; # any module under Omnixient's modules/themes/
+        sshKeys = [ ]; # e.g. [ "ssh-ed25519 AAAA… alice@myhost" ]
+        stateVersion = "24.11"; # release you first installed — never change
+      };
+    in
+    {
+      nixosConfigurations.${settings.hostName} = omni.lib.mkSystem settings.hostName {
+        system = "x86_64-linux";
+        user = settings.user;
+        extraSpecialArgs = { inherit settings; };
+        extraHmArgs = { inherit settings; };
+        modules = [
+          omni.nixosModules.omni # Omnixient desktop/system core
+          ./hosts/myhost # your hardware + option choices
+        ];
+      };
+    };
+}