feat: 重构配置校验为 TypeBox + Ajv + semantic validator，严格禁止未知字段

- 新增 config-contract 模块（TypeBox fragments、Ajv 契约校验、ConfigValidationIssue）
- CheckerDefinition 扩展为含 configKey、schemas、validate 的完整插件接口
- HTTP/Command 各自维护 contract.ts + validate.ts，校验从 resolve 中分离
- resolve 不再承担校验，只做默认值合并和路径/单位解析
- config-loader 流程: unknown → RawProbeConfig → ValidatedProbeConfig → ResolvedConfig
- 导出 probe-config.schema.json，新增 schema/schema:check 脚本
- 更新 DEVELOPMENT.md 新增 1.7 开发新 Checker 完整指引
- 同步更新 4 个 main specs（probe-config、command-checker、expect-body-checkers、checker-runner-abstraction）

src/server/checker/runner/index.ts

@@ -1,10 +1,16 @@
 import { CommandChecker } from "./command/runner";
 import { HttpChecker } from "./http/runner";
-import { checkerRegistry } from "./registry";
+import { CheckerRegistry, checkerRegistry } from "./registry";
 
-export function registerCheckers(): void {
-  checkerRegistry.register(new HttpChecker());
-  checkerRegistry.register(new CommandChecker());
+export function createDefaultCheckerRegistry(): CheckerRegistry {
+  const registry = new CheckerRegistry();
+  registerCheckers(registry);
+  return registry;
+}
+
+export function registerCheckers(registry = checkerRegistry): void {
+  registry.register(new HttpChecker());
+  registry.register(new CommandChecker());
 }
 
 export { checkerRegistry } from "./registry";