feat: add CPU load alert, lower disk/RAM thresholds (SCALE-04)

- Add CpuLoad alert rule: fires when 5min load > 2x core count
- Lower disk usage alert from 90% to 80%
- Lower RAM usage alert from 90% to 80%
- Add num_cpus dependency for runtime core detection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/monitoring/mod.rs

@@ -64,6 +64,7 @@ const MAX_ALERT_HISTORY: usize = 100;
 pub enum AlertRuleKind {
     DiskUsage,
     RamUsage,
+    CpuLoad,
     ContainerCrash,
     BackendErrorSpike,
     SslCertExpiry,
@@ -95,15 +96,21 @@ impl AlertRule {
         vec![
             AlertRule {
                 kind: AlertRuleKind::DiskUsage,
-                threshold: 90.0,
+                threshold: 80.0,
                 enabled: true,
                 description: "Disk usage exceeds threshold".to_string(),
             },
             AlertRule {
                 kind: AlertRuleKind::RamUsage,
-                threshold: 90.0,
+                threshold: 80.0,
                 enabled: true,
-                description: "RAM usage exceeds threshold".to_string(),
+                description: "Total memory usage exceeds threshold".to_string(),
+            },
+            AlertRule {
+                kind: AlertRuleKind::CpuLoad,
+                threshold: 2.0,
+                enabled: true,
+                description: "CPU load exceeds 2x core count for 5 minutes".to_string(),
             },
             AlertRule {
                 kind: AlertRuleKind::ContainerCrash,
@@ -335,6 +342,25 @@ impl MetricsStore {
                         }
                     }
                 }
+                AlertRuleKind::CpuLoad => {
+                    // Alert if 5-min load average exceeds threshold * core count
+                    let cores = num_cpus::get() as f64;
+                    let max_load = rule.threshold * cores;
+                    if snapshot.system.load_avg_5 > max_load {
+                        new_alerts.push(FiredAlert {
+                            id: format!("cpu-{}", ts),
+                            kind: AlertRuleKind::CpuLoad,
+                            message: format!(
+                                "CPU load at {:.1} (threshold: {:.0} = {:.0}x {} cores)",
+                                snapshot.system.load_avg_5, max_load, rule.threshold, cores as u32
+                            ),
+                            value: snapshot.system.load_avg_5,
+                            threshold: max_load,
+                            timestamp: ts,
+                            acknowledged: false,
+                        });
+                    }
+                }
                 AlertRuleKind::BackendErrorSpike => {
                     if snapshot.rpc_latency_ms > rule.threshold {
                         new_alerts.push(FiredAlert {