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/http/execute.ts

@@ -175,12 +175,9 @@ export class HttpChecker implements CheckerDefinition<ResolvedHttpTarget> {
 
   resolve(target: RawTargetConfig, context: ResolveContext): ResolvedHttpTarget {
     const t = target as RawTargetConfig & { http: HttpTargetConfig; type: "http" };
-    const httpDefaults = context.defaults["http"] as
-      | undefined
-      | { headers?: Record<string, string>; maxBodyBytes?: string };
 
     const method = t.http.method ?? "GET";
-    const maxBodyBytes = parseSize(t.http.maxBodyBytes ?? httpDefaults?.maxBodyBytes ?? "100MB");
+    const maxBodyBytes = parseSize(t.http.maxBodyBytes ?? "100MB");
 
     const rawExpect = target.expect as RawHttpExpectConfig | undefined;
     const resolvedExpect: ResolvedHttpExpectConfig = rawExpect
@@ -198,7 +195,7 @@ export class HttpChecker implements CheckerDefinition<ResolvedHttpTarget> {
       group: target.group ?? "default",
       http: {
         body: t.http.body,
-        headers: { ...(httpDefaults?.headers ?? {}), ...(t.http.headers ?? {}) },
+        headers: { ...(t.http.headers ?? {}) },
         ignoreSSL: t.http.ignoreSSL ?? false,
         maxBodyBytes,
         maxRedirects: t.http.maxRedirects ?? 0,