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/config-loader.ts

@@ -1,32 +1,19 @@
 import type {
-  CommandDefaultsConfig,
-  CommandTargetConfig,
   DefaultsConfig,
-  HttpDefaultsConfig,
-  HttpExpectConfig,
-  HttpTargetConfig,
   ProbeConfig,
-  ResolvedCommandTarget,
-  ResolvedHttpTarget,
   ResolvedTarget,
   EngineRuntimeConfig,
   TargetConfig,
-  TargetType,
 } from "./types";
-import { parseSize } from "./size";
-import { resolve } from "node:path";
-import { dirname } from "node:path";
+import { dirname, resolve } from "node:path";
+import { checkerRegistry } from "./runner";
 
 const DEFAULT_HOST = "127.0.0.1";
 const DEFAULT_PORT = 3000;
 const DEFAULT_DATA_DIR = "./data";
 const DEFAULT_INTERVAL = "30s";
 const DEFAULT_TIMEOUT = "10s";
-const DEFAULT_HTTP_METHOD = "GET";
-const DEFAULT_MAX_BODY_BYTES = "100MB";
-const DEFAULT_MAX_OUTPUT_BYTES = "100MB";
 const DEFAULT_MAX_CONCURRENT_CHECKS = 20;
-const SUPPORTED_TYPES: TargetType[] = ["http", "command"];
 
 export interface ResolvedConfig {
   host: string;
@@ -100,73 +87,14 @@ function resolveTarget(
 ): ResolvedTarget {
   const intervalMs = parseDuration(target.interval ?? defaults.interval ?? DEFAULT_INTERVAL);
   const timeoutMs = parseDuration(target.timeout ?? defaults.timeout ?? DEFAULT_TIMEOUT);
-  const group = target.group ?? "default";
 
-  if (target.type === "http") {
-    return resolveHttpTarget(target, defaults.http, intervalMs, timeoutMs, group);
-  }
+  const checker = checkerRegistry.get(target.type);
+  const result = checker.resolve(target, { defaults, configDir, defaultIntervalMs, defaultTimeoutMs });
 
-  return resolveCommandTarget(target, defaults.command, intervalMs, timeoutMs, configDir, group);
-}
+  result.intervalMs = intervalMs;
+  result.timeoutMs = timeoutMs;
 
-function resolveHttpTarget(
-  target: TargetConfig & { type: "http"; http: HttpTargetConfig },
-  httpDefaults: HttpDefaultsConfig | undefined,
-  intervalMs: number,
-  timeoutMs: number,
-  group: string,
-): ResolvedHttpTarget {
-  const maxBodyBytes = parseSize(target.http.maxBodyBytes ?? httpDefaults?.maxBodyBytes ?? DEFAULT_MAX_BODY_BYTES);
-
-  return {
-    type: "http",
-    name: target.name,
-    group,
-    http: {
-      url: target.http.url,
-      method: target.http.method ?? httpDefaults?.method ?? DEFAULT_HTTP_METHOD,
-      headers: { ...(httpDefaults?.headers ?? {}), ...(target.http.headers ?? {}) },
-      body: target.http.body,
-      maxBodyBytes,
-    },
-    intervalMs,
-    timeoutMs,
-    expect: target.expect as HttpExpectConfig | undefined,
-  };
-}
-
-function resolveCommandTarget(
-  target: TargetConfig & { type: "command"; command: CommandTargetConfig },
-  commandDefaults: CommandDefaultsConfig | undefined,
-  intervalMs: number,
-  timeoutMs: number,
-  configDir: string,
-  group: string,
-): ResolvedCommandTarget {
-  const cwd = target.command.cwd ?? commandDefaults?.cwd ?? ".";
-  const resolvedCwd = resolve(configDir, cwd);
-
-  const maxOutputBytes = parseSize(
-    target.command.maxOutputBytes ?? commandDefaults?.maxOutputBytes ?? DEFAULT_MAX_OUTPUT_BYTES,
-  );
-
-  const env = { ...process.env, ...(target.command.env ?? {}) } as Record<string, string>;
-
-  return {
-    type: "command",
-    name: target.name,
-    group,
-    command: {
-      exec: target.command.exec,
-      args: target.command.args ?? [],
-      cwd: resolvedCwd,
-      env,
-      maxOutputBytes,
-    },
-    intervalMs,
-    timeoutMs,
-    expect: target.expect as import("./types").CommandExpectConfig | undefined,
-  };
+  return result;
 }
 
 function validateConfig(config: ProbeConfig): void {
@@ -175,6 +103,7 @@ function validateConfig(config: ProbeConfig): void {
   }
 
   const names = new Set<string>();
+  const supportedTypes = checkerRegistry.supportedTypes;
 
   for (let i = 0; i < config.targets.length; i++) {
     const raw = config.targets[i] as unknown as Record<string, unknown>;
@@ -189,22 +118,8 @@ function validateConfig(config: ProbeConfig): void {
       throw new Error(`target "${name}" 缺少 type 字段`);
     }
 
-    if (!SUPPORTED_TYPES.includes(type as TargetType)) {
-      throw new Error(`target "${name}" 使用不支持的 type: "${type}"，支持: ${SUPPORTED_TYPES.join(", ")}`);
-    }
-
-    if (type === "http") {
-      const http = raw["http"] as Record<string, unknown> | undefined;
-      if (!http?.["url"] || typeof http["url"] !== "string" || (http["url"] as string).trim() === "") {
-        throw new Error(`target "${name}" 缺少 http.url 字段`);
-      }
-    }
-
-    if (type === "command") {
-      const cmd = raw["command"] as Record<string, unknown> | undefined;
-      if (!cmd?.["exec"] || typeof cmd["exec"] !== "string" || (cmd["exec"] as string).trim() === "") {
-        throw new Error(`target "${name}" 缺少 command.exec 字段`);
-      }
+    if (!supportedTypes.includes(type)) {
+      throw new Error(`target "${name}" 使用不支持的 type: "${type}"，支持: ${supportedTypes.join(", ")}`);
     }
 
     const group = raw["group"];