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/checker/runner/registry.test.ts

@@ -2,7 +2,7 @@ import { Type } from "@sinclair/typebox";
 import { describe, expect, test } from "bun:test";
 
 import type { Checker } from "../../../../src/server/checker/runner/types";
-import type { CheckResult, ResolvedTarget } from "../../../../src/server/checker/types";
+import type { CheckResult, ResolvedTargetBase } from "../../../../src/server/checker/types";
 
 import { createDefaultCheckerRegistry } from "../../../../src/server/checker/runner";
 import { CheckerRegistry } from "../../../../src/server/checker/runner/registry";
@@ -11,7 +11,7 @@ function createChecker(type: string): Checker {
   return {
     configKey: type,
     execute: () => Promise.resolve<CheckResult>({} as unknown as CheckResult),
-    resolve: () => ({}) as unknown as ResolvedTarget,
+    resolve: () => ({}) as unknown as ResolvedTargetBase,
     schemas: {
       config: Type.Object({}, { additionalProperties: false }),
       defaults: Type.Object({}, { additionalProperties: false }),