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/registry.test.ts

@@ -15,7 +15,6 @@ function createChecker(type: string): Checker {
     resolve: () => ({}) as unknown as ResolvedTargetBase,
     schemas: {
       config: Type.Object({}, { additionalProperties: false }),
-      defaults: Type.Object({}, { additionalProperties: false }),
       expect: Type.Object({}, { additionalProperties: false }),
     },
     serialize: () => ({ config: "", target: "" }),
@@ -69,11 +68,7 @@ describe("CheckerRegistry", () => {
 
     expect(first.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "icmp", "udp", "llm", "custom"]);
     expect(second.supportedTypes).toEqual(["http", "cmd", "db", "tcp", "icmp", "udp", "llm"]);
-    expect(
-      first.definitions.every(
-        (checker) => checker.schemas.config && checker.schemas.defaults && checker.schemas.expect,
-      ),
-    ).toBe(true);
+    expect(first.definitions.every((checker) => checker.schemas.config && checker.schemas.expect)).toBe(true);
   });
 
   test("默认 registry 注册 icmp type", () => {