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/hosts/myhost/default.nix

@@ -0,0 +1,33 @@
+# Your host. Replace hardware-configuration.nix with your real scan:
+#   sudo nixos-generate-config --show-hardware-config \
+#     > hosts/myhost/hardware-configuration.nix
+{ settings, ... }:
+{
+  imports = [ ./hardware-configuration.nix ];
+
+  # Omnixient behavior. See Omnixient's docs/packs.md for the full pack catalog.
+  omni = {
+    enable = true;
+    desktop.enable = true;
+    user = settings.user;
+    theme = settings.theme;
+
+    packs.development.enable = true;
+    packs.media.enable = true;
+    # packs.gaming.enable = true;
+    # packs.office.enable = true;
+
+    # Opt into sops-nix secrets only if you need them (see Omnixient's
+    # docs/getting-started.md "Enabling secrets"). Off by default.
+    # secrets.enable = true;
+  };
+
+  # System basics Omnixient core leaves to you:
+  networking.hostName = settings.hostName;
+  time.timeZone = settings.timeZone;
+  nix.settings.experimental-features = [
+    "nix-command"
+    "flakes"
+  ];
+  system.stateVersion = settings.stateVersion;
+}