nixos: create 'modules/nixos' folder

Let's consolidate all modules under one path, so that NixOS,
home-manager, and nix-darwin (if I ever end up using it down the line)
would go under the same folder.

modules/nixos/services/tlp/default.nix

@@ -0,0 +1,26 @@
+# TLP power management
+{ config, lib, ... }:
+let
+  cfg = config.my.services.tlp;
+in
+{
+  options.my.services.tlp = {
+    enable = lib.mkEnableOption "TLP power management configuration";
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.tlp = {
+      enable = true;
+
+      settings = {
+        # Set CPU scaling aggressively when power is not an issue
+        CPU_SCALING_GOVERNOR_ON_AC = "performance";
+        CPU_SCALING_GOVERNOR_ON_BAT = "powersave";
+
+        # Keep charge between 60% and 80% to preserve battery life
+        START_CHARGE_THRESH_BAT0 = 60;
+        STOP_CHARGE_THRESH_BAT0 = 80;
+      };
+    };
+  };
+}