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

tests/server/app.test.ts

@@ -7,8 +7,8 @@ import type { HealthResponse, HistoryResponse, SummaryResponse, TargetStatus } f
 
 import { createFetchHandler, type StaticAssets } from "../../src/server/app";
 import { checkerRegistry } from "../../src/server/checker/runner";
-import { CommandChecker } from "../../src/server/checker/runner/command/runner";
-import { HttpChecker } from "../../src/server/checker/runner/http/runner";
+import { CommandChecker } from "../../src/server/checker/runner/command/execute";
+import { HttpChecker } from "../../src/server/checker/runner/http/execute";
 import { ProbeStore } from "../../src/server/checker/store";
 import { rmRetry } from "../helpers";