Initial commit: Elixir MQTT system stats daemon

- MQTT client with configurable broker connection
- Periodic system stats publishing (30s interval)
- Command listening on MQTT topic with logging
- Systemd service configuration
- NixOS module for declarative deployment

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

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

test/system_stats_daemon_test.exs

@@ -0,0 +1,8 @@
+defmodule SystemStatsDaemonTest do
+  use ExUnit.Case
+  doctest SystemStatsDaemon
+
+  test "greets the world" do
+    assert SystemStatsDaemon.hello() == :world
+  end
+end

test/test_helper.exs

@@ -0,0 +1 @@
+ExUnit.start()