refactor: 移除顶层 defaults 配置段，简化为 target 显式字段 > 代码内置默认值

- 移除 DefaultsConfig 类型、ProbeConfig.defaults 字段
- 移除 CheckerSchemas.defaults、ResolveContext.defaults、CheckerValidationInput.defaults
- 更新所有 checker schema/resolve/validate 删除 defaults 合并逻辑
- 更新 config-loader 不再读取传递 defaults
- 更新测试、README、DEVELOPMENT、probes.example.yaml
- 重新生成 probe-config.schema.json（不含 defaults）
- 同步 delta specs 到主规范
- 归档 openspec change

tests/server/checker/runner/icmp/execute.test.ts

@@ -124,7 +124,7 @@ describe("IcmpChecker resolve", () => {
   test("解析默认值", () => {
     const target = checker.resolve(
       { icmp: { host: "10.0.0.1" }, id: "ping", type: "icmp" },
-      { configDir: "/tmp", defaultIntervalMs: 30000, defaults: {}, defaultTimeoutMs: 10000 },
+      { configDir: "/tmp", defaultIntervalMs: 30000, defaultTimeoutMs: 10000 },
     );
     expect(target.icmp).toEqual({ count: 3, host: "10.0.0.1", packetSize: 56 });
     expect(target.group).toBe("default");