refactor: 引入 Checker 统一接口与 Runner 抽象机制

定义 Checker 接口（resolve/execute/serialize）和 CheckerRegistry
注册中心，消除 engine/config-loader/store 中硬编码类型分支。
按 checker 类型分子包（runner/http/、runner/command/），提取
共享 expect 到 runner/shared/。超时控制通过引擎注入 AbortSignal。
CheckFailure.phase 从联合类型改为 string。配置校验下沉到各
Checker.resolve() 内部。

新增 checker-runner-abstraction spec，更新 DEVELOPMENT.md。

tests/server/checker/config-loader.test.ts

@@ -4,6 +4,20 @@ import { readRuntimeConfig } from "../../../src/server/config";
 import { mkdir, rm, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
+import { checkerRegistry } from "../../../src/server/checker/runner";
+import { HttpChecker } from "../../../src/server/checker/runner/http/runner";
+import { CommandChecker } from "../../../src/server/checker/runner/command/runner";
+
+function ensureRegistered() {
+  if (!checkerRegistry.supportedTypes.includes("http")) {
+    checkerRegistry.register(new HttpChecker());
+    checkerRegistry.register(new CommandChecker());
+  }
+}
+
+beforeAll(() => {
+  ensureRegistered();
+});
 
 describe("parseDuration", () => {
   test("解析秒", () => {