feat: implement cross-platform metric collector with gopsutil, rename module to nexusrmm.local/agent

Agent/internal/collector/collector_test.go

@@ -0,0 +1,19 @@
+package collector
+
+import "testing"
+
+func TestCollect(t *testing.T) {
+	metrics, err := Collect()
+	if err != nil {
+		t.Fatalf("Collect() error: %v", err)
+	}
+	if metrics.CPUPercent < 0 || metrics.CPUPercent > 100 {
+		t.Errorf("CPU percent out of range: %f", metrics.CPUPercent)
+	}
+	if metrics.MemoryTotal == 0 {
+		t.Error("MemoryTotal should not be 0")
+	}
+	if len(metrics.Disks) == 0 {
+		t.Error("Expected at least one disk")
+	}
+}