Clean up Nix configuration and remove old files

- Move Nix package and module to nix/ directory for better organization
- Remove old Nix files (systant.nix, systant-old.nix, etc.)
- Remove debug script and systemd service file
- Update config files for new Nix structure
- Add CLAUDE.md with project documentation

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

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

config/config.exs

@@ -1,15 +1,5 @@
 import Config
 
-config :systant, Systant.MqttClient,
-  host: "mqtt.home",
-  port: 1883,
-  client_id: "systant",
-  username: "mqtt",
-  password: "pleasework",
-  stats_topic_base: "systant",  # Will become systant/{hostname}/stats
-  command_topic_base: "systant", # Will become systant/{hostname}/commands
-  publish_interval: 30_000
-
 config :logger, :console,
   format: "$time $metadata[$level] $message\n",
   metadata: [:request_id]

config/runtime.exs

@@ -8,11 +8,11 @@ end
 
 # Runtime configuration that can use environment variables
 config :systant, Systant.MqttClient,
-  host: System.get_env("SYSTANT_MQTT_HOST", "localhost"),
+  host: System.get_env("SYSTANT_MQTT_HOST", "mqtt.home"),
   port: String.to_integer(System.get_env("SYSTANT_MQTT_PORT", "1883")),
   client_id: System.get_env("SYSTANT_CLIENT_ID", "systant"),
   username: System.get_env("SYSTANT_MQTT_USERNAME"),
   password: System.get_env("SYSTANT_MQTT_PASSWORD"),
   stats_topic: System.get_env("SYSTANT_STATS_TOPIC", "systant/#{hostname}/stats"),
   command_topic: System.get_env("SYSTANT_COMMAND_TOPIC", "systant/#{hostname}/commands"),
-  publish_interval: String.to_integer(System.get_env("SYSTANT_PUBLISH_INTERVAL", "30000"))
+  publish_interval: String.to_integer(System.get_env("SYSTANT_PUBLISH_INTERVAL", "30000"))