feat: 新增本机 CPU checker

- 新增 type: cpu checker，基于 os.cpus() 两次快照计算 CPU 使用率
- 配置项：sampleDuration（默认 1s）、includePerCore（默认 false）
- expect 字段：usagePercent、idlePercent、maxCoreUsagePercent、minCoreUsagePercent、durationMs
- idlePercent 与 usagePercent 互补恒等于 100，百分比范围 0-100
- logicalCoreCount 仅输出到 observation，不作为 expect 字段
- 不暴露 userPercent / systemPercent
- 语义校验禁止 sampleDuration >= timeout
- 支持 AbortSignal 超时取消
- 完整测试覆盖：schema、validate、normalize、resolve、calculate、execute、expect、config-loader
- 新增用户文档 docs/user/checkers/cpu.md
- 更新 checker 索引、配置类型列表、示例配置和 schema

tests/server/checker/runner/registry.test.ts

@@ -73,8 +73,20 @@ describe("CheckerRegistry", () => {
     const second = createDefaultCheckerRegistry();
     first.register(createChecker("custom"));
 
-    expect(first.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "icmp", "udp", "llm", "dns", "ws", "custom"]);
-    expect(second.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "icmp", "udp", "llm", "dns", "ws"]);
+    expect(first.supportedTypes).toEqual([
+      "http",
+      "cmd",
+      "db",
+      "tcp",
+      "icmp",
+      "udp",
+      "llm",
+      "dns",
+      "ws",
+      "cpu",
+      "custom",
+    ]);
+    expect(second.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "icmp", "udp", "llm", "dns", "ws", "cpu"]);
     expect(
       first.definitions.every((checker) => checker.schemas.authoring.config && checker.schemas.normalized.expect),
     ).toBe(true);