feat(flake): expose nixosModules.omnixy core aggregate

Extract the reusable core module list into modules/default.nix (single
source of truth) and expose it as nixosModules.omnixy / .default.
configuration.nix now imports ./modules instead of listing each module,
so this repo's hosts and external consumers share one definition. Theme
stays a settings.theme-driven import inside the aggregate. Packs,
dev-env, sops and home-manager remain injected by mkSystem.

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

modules/default.nix

@@ -0,0 +1,48 @@
+# Omnixient core module aggregate.
+#
+# The reusable Omnixient module set as a single importable module, used from
+# one source of truth in two places:
+#   - configuration.nix imports it for this repo's own hosts;
+#   - it's exposed as `nixosModules.omni` (flake.nix) so an external
+#     consumer can `imports = [ omni.nixosModules.omni ]` from their
+#     own flake without forking.
+#
+# Deliberately NOT here:
+#   - modules/packs and modules/dev-env — injected separately by
+#     lib/mksystem.nix (packs unconditionally, dev-env opt-in);
+#   - hardware-configuration.nix — per-host;
+#   - the sops-nix / home-manager modules — injected by mkSystem.
+#
+# The active theme is selected from `settings.theme` (a specialArg
+# provided by mkSystem), matching the prior configuration.nix behavior.
+{
+  imports = [
+    ./lib.nix
+    ./core.nix
+    ./colors.nix
+    ./boot.nix
+    ./security.nix
+    ./secrets.nix
+    ./fastfetch.nix
+    ./walker.nix
+    ./scripts.nix
+    ./menus.nix
+    ./desktop/hyprland.nix
+    ./packages.nix
+    ./development.nix
+    ./mcp.nix
+    ./users.nix
+    ./services.nix
+    ./hardware
+
+    # Theme: import the colorscheme module named by settings.theme. Kept
+    # as a settings-driven dynamic import (rather than importing all 12
+    # and guarding each) so the closure stays minimal.
+    (
+      { settings, ... }:
+      {
+        imports = [ ./themes/${settings.theme}.nix ];
+      }
+    )
+  ];
+}