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/payload.rs

@@ -0,0 +1,30 @@
+// stub — implémenté dans la Task 3
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, Debug, Default)]
+pub struct AgentMetrics {
+    pub hostname: String,
+    pub ip: String,
+    pub status: String,
+    pub cpu_percent: Option<f32>,
+    pub memory_used: Option<u64>,
+    pub memory_free: Option<u64>,
+    pub memory_total: Option<u64>,
+    pub hdd_used: Option<u64>,
+    pub hdd_free: Option<u64>,
+    pub hdd_total: Option<u64>,
+    pub uptime: Option<u64>,
+    pub network_rx: Option<u64>,
+    pub network_tx: Option<u64>,
+    pub temperature: Option<f32>,
+    pub smart: Option<SmartMetrics>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct SmartMetrics {
+    pub passed: bool,
+    pub temperature: Option<i64>,
+    pub reallocated_sectors: Option<i64>,
+    pub power_on_hours: Option<i64>,
+    pub wear_level: Option<i64>,
+}