refactor: checker 模块内聚化 — 每个 checker 自包含于独立目录

将 checker 架构重构为完全内聚模式：每个 checker 目录包含自身的
types、schema、validate、execute、expect 和 index，新增 checker
只需创建一个目录并在 runner/index.ts 添加一行注册。

主要变更：
- runner/shared/ 拆分：断言基础设施迁入 checker/expect/，
  body.ts 迁入 http/，text.ts 迁入 command/
- config-contract/ 重命名为 schema/，schema.ts → builder.ts
- size.ts + parseDuration 合并为 utils.ts
- 顶层 types.ts 改为 base interface + index signature，
  checker 专属类型下沉到各自 types.ts
- runner/index.ts 改为显式数组注册模式
- 更新 DEVELOPMENT.md 项目结构和开发新 Checker 指南

src/server/checker/runner/index.ts

@@ -1,16 +1,15 @@
-import { CommandChecker } from "./command/runner";
-import { HttpChecker } from "./http/runner";
-import { CheckerRegistry, checkerRegistry } from "./registry";
+import { CommandChecker } from "./command";
+import { HttpChecker } from "./http";
+import { CheckerRegistry } from "./registry";
+
+const checkers = [new HttpChecker(), new CommandChecker()];
 
 export function createDefaultCheckerRegistry(): CheckerRegistry {
   const registry = new CheckerRegistry();
-  registerCheckers(registry);
+  for (const checker of checkers) {
+    registry.register(checker);
+  }
   return registry;
 }
 
-export function registerCheckers(registry = checkerRegistry): void {
-  registry.register(new HttpChecker());
-  registry.register(new CommandChecker());
-}
-
-export { checkerRegistry } from "./registry";
+export const checkerRegistry = createDefaultCheckerRegistry();