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/shared/duplicate-header-key.test.ts

@@ -7,7 +7,7 @@ import { validateHttpConfig } from "../../../../../src/server/checker/runner/htt
 import { validateLlmConfig } from "../../../../../src/server/checker/runner/llm/validate";
 
 function input(target: Record<string, unknown>): CheckerValidationInput {
-  return { defaults: {}, targets: [target as CheckerValidationInput["targets"][number]] };
+  return { targets: [target as CheckerValidationInput["targets"][number]] };
 }
 
 describe("HTTP/LLM headers reject case-insensitive duplicate keys", () => {

tests/server/checker/runner/shared/value-matcher-shorthand.test.ts

@@ -11,7 +11,7 @@ import { validateTcpConfig } from "../../../../../src/server/checker/runner/tcp/
 import { validateUdpConfig } from "../../../../../src/server/checker/runner/udp/validate";
 
 function input(target: Record<string, unknown>): CheckerValidationInput {
-  return { defaults: {}, targets: [target as CheckerValidationInput["targets"][number]] };
+  return { targets: [target as CheckerValidationInput["targets"][number]] };
 }
 
 describe("ValueMatcher primitive shorthand in checker validators", () => {