Fix COOKIE file missing in Nix build

- Add postInstall hook to ensure COOKIE file exists
- Include ERTS and disable beam stripping in release config
- Handle mixRelease differences from local builds

This ensures the Elixir release works properly when built via Nix.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

mix.exs

@@ -33,7 +33,9 @@ defmodule SystemStatsDaemon.MixProject do
       systant: [
         include_executables_for: [:unix],
         applications: [runtime_tools: :permanent],
-        cookie: "systant-cookie-change-in-production"
+        cookie: "systant-cookie-change-in-production",
+        include_erts: true,
+        strip_beams: false
       ]
     ]
   end

systant.nix

@@ -6,17 +6,25 @@ beamPackages.mixRelease rec {
 
   src = fetchgit {
     url = "https://git.ryanpandya.com/ryan/systant.git";
-    rev = "92fc90e3b470dd2d11ba3a84745e33195e8e9db3";
-    sha256 = lib.fakeSha256; # Replace with actual hash after first build attempt
+    rev = "9d8ad1890b682a58aaa27406a7f28337c4c67d5f";
+    sha256 = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
   };
 
   # Mix dependencies will be automatically fetched and cached by Nix
   mixFodDeps = beamPackages.fetchMixDeps {
     pname = "systant-mix-deps";
     inherit src version;
-    sha256 = lib.fakeSha256; # Will get this from first build failure
+    sha256 = "sha256-AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA=";
   };
 
+  # Ensure COOKIE file exists
+  postInstall = ''
+    if [ ! -f $out/releases/COOKIE ]; then
+      mkdir -p $out/releases
+      echo "systant-cookie-change-in-production" > $out/releases/COOKIE
+    fi
+  '';
+
   meta = with lib; {
     description = "Systant - System stats MQTT daemon for monitoring system metrics";
     homepage = "https://git.ryanpandya.com/ryan/systant";