feat(agent): setup Cargo.toml et structure complète

Initialise la structure de l'agent Rust avec tous les modules stub :
config, payload, metrics (cpu/memory/disk/network/uptime/temperature/smart),
transport (udp/mqtt). Corrige les features sysinfo 0.30 (pas de feature
disk/networks séparées). Compile sans erreurs.

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

agent/src/transport/mqtt.rs

@@ -0,0 +1,22 @@
+use crate::config::MqttConfig;
+use std::sync::mpsc::Sender;
+
+pub enum MqttIncoming {
+    ConfigUpdate(Vec<u8>),
+}
+
+pub struct MqttClient;
+
+impl MqttClient {
+    pub fn clone(&self) -> Self {
+        MqttClient
+    }
+}
+
+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);
+    client
+}
+
+pub fn publish_metrics(_client: &rumqttc::Client, _base: &str, _hostname: &str, _json: &str) {}