feat: add fleet configuration options module

- Create fleet/fleet-option.nix to define athenix.fleet and athenix.hwTypes options
- Use lib.evalModules to evaluate inventory separately from host configs
- Enable external overrides of fleet and hardware types via lib.mkForce
- Fix infinite recursion issue by avoiding config references in imports

fleet/fleet-option.nix

@@ -0,0 +1,66 @@
+# ============================================================================
+# Fleet Option Definition
+# ============================================================================
+# This module only defines the athenix.fleet option without any dependencies.
+# Used by fleet.nix to evaluate inventory data without circular dependencies.
+{ inputs, ... }:
+{ lib, ... }:
+let
+  fleetDefinition = lib.mkOption {
+    description = "Hardware types definitions for the fleet.";
+    type = lib.types.attrsOf (lib.types.submodule ({ name, ...}: {
+      options = {
+        type = lib.mkOption {
+          type = lib.types.oneOf [
+            lib.types.str
+            lib.types.listOf lib.types.str
+          ];
+          default = name;
+          description = "Type(s) of system configuration for this device.";
+        };
+        system = lib.mkOption {
+          type = lib.types.str;
+          default = "x86_64-linux";
+          description = "NixOS system architecture for this hardware type.";
+        };
+        devices = lib.mkOption {
+          type = lib.types.oneOf [
+            lib.types.int
+            (lib.types.attrsOf 
+              (lib.types.submodule ({ name, ... }: {
+                freeformType = lib.types.attrs;
+              })
+            ))
+          ];
+        };
+        count = lib.mkOption {
+          type = lib.types.int;
+          default = 0;
+          description = "Number of devices of this type to create.";
+        };
+        defaultCount = lib.mkOption {
+          type = lib.types.int;
+          default = 0;
+          description = "Default number of devices to create with default configurations and numbered hostnames.";
+        };
+        overrides = lib.mkOption {
+          type = lib.types.attrs;
+          default = { };
+          description = "Overrides to apply to all devices of this type.";
+        };
+      };
+    }));
+  };
+in
+{
+  options.athenix = {
+    fleet = fleetDefinition;
+    hwTypes = lib.mkOption {
+      description = "Hardware types definitions for the fleet.";
+      type = lib.types.attrs;
+    };
+  };
+
+  config.athenix.fleet = lib.mkDefault (import ../inventory.nix);
+  config.athenix.hwTypes = lib.mkDefault (import ../hw { inherit inputs; });
+}