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/runner/command/expect.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, test } from "bun:test";
+import { checkExitCode } from "../../../../../src/server/checker/runner/command/expect";
+
+describe("checkExitCode", () => {
+  test("exitCode 在允许列表中匹配成功", () => {
+    const r = checkExitCode(0, [0]);
+    expect(r.matched).toBe(true);
+    expect(r.failure).toBeNull();
+  });
+
+  test("exitCode 不在允许列表中匹配失败", () => {
+    const r = checkExitCode(1, [0]);
+    expect(r.matched).toBe(false);
+    expect(r.failure!.phase).toBe("exitCode");
+    expect(r.failure!.kind).toBe("mismatch");
+    expect(r.failure!.expected).toEqual([0]);
+    expect(r.failure!.actual).toBe(1);
+  });
+
+  test("多个允许退出码", () => {
+    expect(checkExitCode(0, [0, 1]).matched).toBe(true);
+    expect(checkExitCode(1, [0, 1]).matched).toBe(true);
+    expect(checkExitCode(2, [0, 1]).matched).toBe(false);
+  });
+});