feat(system): enable ddcci for external monitor brightness control

modules/nixos/hardware.nix

@@ -136,6 +136,17 @@ in
         '';
       };
     };
+
+    i2c = {
+      enable = lib.mkEnableOption ''
+        I2C devices support. Enables access to /dev/i2c-* (useful for RGB
+        controllers, sensors, and DDC/CI monitor control)'';
+
+      ddcci = lib.mkEnableOption ''
+        the ddcci-driver kernel module to expose external monitors as standard
+        backlight devices via DDC/CI. This allows brightness keys and swayosd
+        to natively control external displays'';
+    };
   };
 
   config = lib.mkMerge [
@@ -221,6 +232,17 @@ in
       };
     })
 
+    # ── I2C / DDC/CI ───────────────────────────────────────────────────
+    (lib.mkIf cfg.i2c.enable {
+      hardware.i2c.enable = true;
+    })
+    (lib.mkIf cfg.i2c.ddcci {
+      # The driver needs I2C underneath it
+      hardware.i2c.enable = true;
+      boot.extraModulePackages = [ config.boot.kernelPackages.ddcci-driver ];
+      boot.kernelModules = [ "ddcci_backlight" ];
+    })
+
     # ── Sanity ─────────────────────────────────────────────────────────
     {
       assertions = [