feat(agent): métriques CPU, RAM, réseau, uptime

Implémentation complète des 4 modules sysinfo 0.30 (sans default-features).
- cpu: moyenne sur tous les cœurs via refresh_cpu_usage()
- memory: used/free/total via System
- network: total_received/total_transmitted via Networks
- uptime: System::uptime() statique
10 tests unitaires passent.

agent/src/metrics/memory.rs

@@ -1,3 +1,20 @@
-pub fn get(_sys: &sysinfo::System) -> (u64, u64, u64) {
-    (0, 0, 0)
+use sysinfo::System;
+
+pub fn get(sys: &System) -> (u64, u64, u64) {
+    (sys.used_memory(), sys.free_memory(), sys.total_memory())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use sysinfo::System;
+
+    #[test]
+    fn test_memory_coherent() {
+        let mut sys = System::new();
+        sys.refresh_memory();
+        let (used, free, total) = get(&sys);
+        assert!(total > 0, "Total mémoire nul");
+        assert!(used + free <= total + 1024 * 1024, "used + free > total");
+    }
 }