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。

scripts/build.ts

@@ -103,9 +103,12 @@ import { ProbeStore } from "../src/server/checker/store";
 import { ProbeEngine } from "../src/server/checker/engine";
 import { startServer } from "../src/server/server";
 import { readRuntimeConfig } from "../src/server/config";
+import { registerCheckers } from "../src/server/checker/runner";
 import { staticAssets } from "./static-assets";
 
 async function main() {
+  registerCheckers();
+
   const { configPath } = readRuntimeConfig();
   const config = await loadConfig(configPath);