Add hostname-aware MQTT topics

- Topics now default to systant/{hostname}/stats and systant/{hostname}/commands
- Runtime config automatically includes system hostname in topics
- NixOS module defaults use config.networking.hostName
- Supports multiple hosts without topic conflicts
- Environment variables can still override if needed

Example: systant/orion/stats, systant/server2/stats, etc.

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

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

config/config.exs

@@ -6,10 +6,10 @@ config :systant, Systant.MqttClient,
   client_id: "systant",
   username: "mqtt",
   password: "pleasework",
-  stats_topic: "system/stats",
-  command_topic: "system/commands",
+  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]
+  metadata: [:request_id]

config/runtime.exs

@@ -1,5 +1,11 @@
 import Config
 
+# Get hostname for topic construction
+hostname = case :inet.gethostname() do
+  {:ok, hostname} -> List.to_string(hostname)
+  _ -> "unknown"
+end
+
 # Runtime configuration that can use environment variables
 config :systant, Systant.MqttClient,
   host: System.get_env("SYSTANT_MQTT_HOST", "localhost"),
@@ -7,6 +13,6 @@ config :systant, Systant.MqttClient,
   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", "system/stats"),
-  command_topic: System.get_env("SYSTANT_COMMAND_TOPIC", "system/commands"),
+  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"))