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

modules/nixos/default.nix

@@ -145,6 +145,9 @@ in
     # between earlyoom (true) and smartd (false).
     services.systembus-notify.enable = true;
 
+    # Enable I2C and DDC/CI by default for external monitor brightness control.
+    nomarchy.hardware.i2c.ddcci = lib.mkDefault true;
+
     networking.networkmanager.enable = lib.mkDefault true;
     # OpenVPN support for the VPN menu's import/connect flow (nomarchy-vpn).
     # WireGuard needs no plugin (NetworkManager imports wg .conf natively);

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 = [