feat(agent): transport MQTT birth/LWT/subscribe config

Implémente le transport MQTT complet avec rumqttc 0.24 :
- Birth message (online) au ConnAck + subscribe config topic
- Last Will Testament (offline) configuré au connect
- Réception config agent via topic {base}/{hostname}/config
- publish_metrics helper pour envoyer les métriques
- Transport::Tcp forcé pour broker sans TLS (malgré use-native-tls feature)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent/src/transport/mqtt.rs

@@ -1,22 +1,82 @@
-use crate::config::MqttConfig;
+use rumqttc::{Client, LastWill, MqttOptions, QoS, Event, Packet, Transport};
 use std::sync::mpsc::Sender;
+use std::time::Duration;
+use crate::config::MqttConfig;
 
 pub enum MqttIncoming {
     ConfigUpdate(Vec<u8>),
 }
 
-pub struct MqttClient;
+pub fn start(
+    hostname: &str,
+    cfg: &MqttConfig,
+    incoming_tx: Sender<MqttIncoming>,
+) -> Client {
+    let status_topic = format!("{}/{}/status", cfg.topic_base, hostname);
+    let config_topic = format!("{}/{}/config", cfg.topic_base, hostname);
 
-impl MqttClient {
-    pub fn clone(&self) -> Self {
-        MqttClient
+    let mut opts = MqttOptions::new(
+        format!("nanometrics-{}", hostname),
+        &cfg.host,
+        cfg.port,
+    );
+    opts.set_keep_alive(Duration::from_secs(30));
+    // Forcer TCP non-TLS (use-native-tls est activé en feature mais le broker est sans TLS)
+    opts.set_transport(Transport::Tcp);
+
+    if cfg.last_will {
+        opts.set_last_will(LastWill::new(
+            &status_topic,
+            "offline",
+            QoS::AtLeastOnce,
+            true,
+        ));
     }
-}
 
-pub fn start(_hostname: &str, _cfg: &MqttConfig, _tx: Sender<MqttIncoming>) -> rumqttc::Client {
-    let opts = rumqttc::MqttOptions::new("stub", "localhost", 1883);
-    let (client, _conn) = rumqttc::Client::new(opts, 1);
+    let (client, mut connection) = Client::new(opts, 16);
+
+    let client_clone = client.clone();
+    let config_topic_clone = config_topic.clone();
+    let status_topic_clone = status_topic.clone();
+    let birth = cfg.birth_message;
+
+    std::thread::spawn(move || {
+        for event in connection.iter() {
+            match event {
+                Ok(Event::Incoming(Packet::ConnAck(_))) => {
+                    if birth {
+                        let _ = client_clone.publish(
+                            &status_topic_clone,
+                            QoS::AtLeastOnce,
+                            true,
+                            "online",
+                        );
+                    }
+                    let _ = client_clone.subscribe(
+                        &config_topic_clone,
+                        QoS::AtLeastOnce,
+                    );
+                }
+                Ok(Event::Incoming(Packet::Publish(p))) => {
+                    if p.topic == config_topic_clone {
+                        let _ = incoming_tx.send(MqttIncoming::ConfigUpdate(
+                            p.payload.to_vec(),
+                        ));
+                    }
+                }
+                Err(e) => {
+                    eprintln!("[mqtt] erreur: {}", e);
+                    std::thread::sleep(Duration::from_secs(5));
+                }
+                _ => {}
+            }
+        }
+    });
+
     client
 }
 
-pub fn publish_metrics(_client: &rumqttc::Client, _base: &str, _hostname: &str, _json: &str) {}
+pub fn publish_metrics(client: &Client, topic_base: &str, hostname: &str, json: &str) {
+    let topic = format!("{}/{}/metrics", topic_base, hostname);
+    let _ = client.publish(topic, QoS::AtMostOnce, false, json);
+}