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。

src/server/checker/engine.ts

@@ -1,7 +1,6 @@
 import type { CheckResult, ResolvedTarget } from "./types";
 import type { ProbeStore } from "./store";
-import { runHttpCheck } from "./fetcher";
-import { runCommandCheck } from "./command-runner";
+import { checkerRegistry } from "./runner";
 import { groupBy, Semaphore } from "es-toolkit";
 
 export class ProbeEngine {
@@ -61,11 +60,14 @@ export class ProbeEngine {
   }
 
   private async runCheck(target: ResolvedTarget): Promise<CheckResult> {
-    switch (target.type) {
-      case "http":
-        return runHttpCheck(target);
-      case "command":
-        return runCommandCheck(target);
+    const checker = checkerRegistry.get(target.type);
+    const controller = new AbortController();
+    const timeoutId = setTimeout(() => controller.abort(), target.timeoutMs);
+
+    try {
+      return await checker.execute(target, { signal: controller.signal });
+    } finally {
+      clearTimeout(timeoutId);
     }
   }