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/udp/execute.test.ts

@@ -334,7 +334,7 @@ describe("UdpChecker resolve", () => {
     const checker = new UdpChecker();
     const target = checker.resolve(
       { id: "test", type: "udp", udp: { host: "127.0.0.1", port: 9000 } },
-      { configDir: "/tmp", defaultIntervalMs: 30000, defaults: {}, defaultTimeoutMs: 10000 },
+      { configDir: "/tmp", defaultIntervalMs: 30000, defaultTimeoutMs: 10000 },
     );
     expect(target.udp.payload).toBe("");
     expect(target.udp.encoding).toBe("text");
@@ -344,32 +344,11 @@ describe("UdpChecker resolve", () => {
     expect(target.expect).toEqual({ responded: true });
   });
 
-  it("should use defaults.udp for missing fields", () => {
-    const checker = new UdpChecker();
-    const target = checker.resolve(
-      { id: "test", type: "udp", udp: { host: "127.0.0.1", port: 9000 } },
-      {
-        configDir: "/tmp",
-        defaultIntervalMs: 30000,
-        defaults: { udp: { encoding: "hex", maxResponseBytes: "8KB", responseEncoding: "hex" } },
-        defaultTimeoutMs: 10000,
-      },
-    );
-    expect(target.udp.encoding).toBe("hex");
-    expect(target.udp.responseEncoding).toBe("hex");
-    expect(target.udp.maxResponseBytes).toBe(8192);
-  });
-
   it("should override defaults with target-level config", () => {
     const checker = new UdpChecker();
     const target = checker.resolve(
       { id: "test", type: "udp", udp: { encoding: "base64", host: "127.0.0.1", port: 9000 } },
-      {
-        configDir: "/tmp",
-        defaultIntervalMs: 30000,
-        defaults: { udp: { encoding: "hex" } },
-        defaultTimeoutMs: 10000,
-      },
+      { configDir: "/tmp", defaultIntervalMs: 30000, defaultTimeoutMs: 10000 },
     );
     expect(target.udp.encoding).toBe("base64");
   });