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>

config/config.exs

@@ -0,0 +1,15 @@
+import Config
+
+config :system_stats_daemon, SystemStatsDaemon.MqttClient,
+  host: "mqtt.home",
+  port: 1883,
+  client_id: "system_stats_daemon",
+  username: "mqtt",
+  password: "pleasework",
+  stats_topic: "system/stats",
+  command_topic: "system/commands",
+  publish_interval: 30_000
+
+config :logger, :console,
+  format: "$time $metadata[$level] $message\n",
+  metadata: [:request_id]