fix(alert-rules): HostHighCpuLoad 獨立規則，停止 kubectl scale unknown

根因: HostHighCpuLoad 是 node_exporter host 告警，無 pod/deployment label
      被分到 K8s high_cpu 規則 → {target}=unknown → auto-repair 安全攔截

修復: 新增 host_cpu_high 規則 (priority=45)，NO_ACTION + 正確描述
     high_cpu 規則移除 HostHighCpuLoad/NodeCPUUsageHigh

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

apps/api/alert_rules.yaml

@@ -107,16 +107,36 @@ rules:
           command: "kubectl autoscale deployment {target} --memory-percent=80 --min=2 --max=5 -n {namespace}"
       reasoning: "[規則匹配] Pod OOMKilled 後 ReplicaSet 將自動重建，但需同步修正資源配置防止復發。"
 
+  # 2026-04-12 ogt: Host CPU 告警獨立規則 — node_exporter 告警無 pod/deployment label
+  #   原本放在 high_cpu 規則導致 {target}="unknown" → auto-repair 安全攔截
+  #   host 告警只能通知，不能 kubectl scale
+  - id: host_cpu_high
+    priority: 45
+    description: Host 主機 CPU 使用率過高 (node_exporter，非 K8s workload)
+    match:
+      alertname:
+        - HostHighCpuLoad
+        - NodeCPUUsageHigh
+        - NodeHighCpuLoad
+    response:
+      action_title: "Host {host} CPU 過高 — 需排查高 CPU 進程"
+      description: "⚠️ 主機 {host} CPU 使用率超標。此為主機層告警，需 SSH 登入排查 (top / ps aux)。常見原因: Ollama 推理、DB 查詢、K3s GC。"
+      suggested_action: NO_ACTION
+      kubectl_command: ""
+      estimated_downtime: "N/A"
+      risk: low
+      responsibility: INFRA
+      reasoning: "[規則匹配] 主機 CPU 告警無法自動修復，需人工確認高 CPU 進程後決策。"
+
   - id: high_cpu
     priority: 40
-    description: Pod/Node CPU 使用率過高
+    description: K8s Pod/Deployment CPU 使用率過高
     match:
       # 2026-04-10 Claude Sonnet 4.6: Phase 2 飛輪修復 — 補齊 Prometheus alertname 變體
+      # 2026-04-12 ogt: 移除 HostHighCpuLoad/NodeCPUUsageHigh → 已獨立為 host_cpu_high 規則
       alertname:
         - HighCPUUsage
         - ContainerCpuUsageSecondsTotal
-        - HostHighCpuLoad
-        - NodeCPUUsageHigh
         - CPUThrottlingHigh
         - KubeCPUOvercommit
       alert_type: