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

src/server/checker/runner/udp/execute.ts

@@ -2,13 +2,7 @@ import { isError } from "es-toolkit";
 
 import type { CheckResult, RawTargetConfig } from "../../types";
 import type { CheckerContext, CheckerDefinition, CheckerValidationInput, ResolveContext } from "../types";
-import type {
-  RawUdpExpectConfig,
-  ResolvedUdpExpectConfig,
-  ResolvedUdpTarget,
-  UdpDefaultsConfig,
-  UdpTargetConfig,
-} from "./types";
+import type { RawUdpExpectConfig, ResolvedUdpExpectConfig, ResolvedUdpTarget, UdpTargetConfig } from "./types";
 
 import { resolveContentExpectations } from "../../expect/content";
 import { errorFailure } from "../../expect/failure";
@@ -304,13 +298,10 @@ export class UdpChecker implements CheckerDefinition<ResolvedUdpTarget> {
 
   resolve(target: RawTargetConfig, context: ResolveContext): ResolvedUdpTarget {
     const t = target as RawTargetConfig & { type: "udp"; udp: UdpTargetConfig };
-    const udpDefaults = context.defaults["udp"] as UdpDefaultsConfig | undefined;
 
-    const encoding = t.udp.encoding ?? udpDefaults?.encoding ?? "text";
-    const responseEncoding = t.udp.responseEncoding ?? udpDefaults?.responseEncoding ?? "text";
-    const maxResponseBytes = parseSize(
-      t.udp.maxResponseBytes ?? udpDefaults?.maxResponseBytes ?? DEFAULT_MAX_RESPONSE_BYTES,
-    );
+    const encoding = t.udp.encoding ?? "text";
+    const responseEncoding = t.udp.responseEncoding ?? "text";
+    const maxResponseBytes = parseSize(t.udp.maxResponseBytes ?? DEFAULT_MAX_RESPONSE_BYTES);
 
     const rawExpect = target.expect as RawUdpExpectConfig | undefined;
     const resolvedExpect: ResolvedUdpExpectConfig = rawExpect