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/command/validate.ts

@@ -1,9 +1,9 @@
-import type { ConfigValidationIssue } from "../../config-contract/issues";
+import type { ConfigValidationIssue } from "../../schema/issues";
 import type { CheckerValidationInput } from "../types";
 
-import { issue, joinPath } from "../../config-contract/issues";
-import { parseSize } from "../../size";
-import { validateTextRules } from "../shared/validate";
+import { validateOperatorObject } from "../../expect/validate-operator";
+import { issue, joinPath } from "../../schema/issues";
+import { parseSize } from "../../utils";
 
 export function validateCommandConfig(input: CheckerValidationInput): ConfigValidationIssue[] {
   const issues: ConfigValidationIssue[] = [];
@@ -91,3 +91,8 @@ function validateSizeValue(value: number | string, path: string, targetName?: st
     return [issue("invalid-size", path, error instanceof Error ? error.message : "size 格式不合法", targetName)];
   }
 }
+
+function validateTextRules(rules: unknown, path: string, targetName?: string): ConfigValidationIssue[] {
+  if (!Array.isArray(rules)) return [issue("invalid-type", path, "必须为数组", targetName)];
+  return rules.flatMap((rule, index) => validateOperatorObject(rule, `${path}[${index}]`, targetName));
+}