refactor: checker 模块内聚化 — 每个 checker 自包含于独立目录

将 checker 架构重构为完全内聚模式：每个 checker 目录包含自身的
types、schema、validate、execute、expect 和 index，新增 checker
只需创建一个目录并在 runner/index.ts 添加一行注册。

主要变更：
- runner/shared/ 拆分：断言基础设施迁入 checker/expect/，
  body.ts 迁入 http/，text.ts 迁入 command/
- config-contract/ 重命名为 schema/，schema.ts → builder.ts
- size.ts + parseDuration 合并为 utils.ts
- 顶层 types.ts 改为 base interface + index signature，
  checker 专属类型下沉到各自 types.ts
- runner/index.ts 改为显式数组注册模式
- 更新 DEVELOPMENT.md 项目结构和开发新 Checker 指南

tests/server/checker/store.test.ts

@@ -3,11 +3,13 @@ import { mkdir } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 
-import type { CheckFailure, ResolvedTarget } from "../../../src/server/checker/types";
+import type { ResolvedCommandTarget } from "../../../src/server/checker/runner/command/types";
+import type { ResolvedHttpTarget } from "../../../src/server/checker/runner/http/types";
+import type { CheckFailure } from "../../../src/server/checker/types";
 
 import { checkerRegistry } from "../../../src/server/checker/runner";
-import { CommandChecker } from "../../../src/server/checker/runner/command/runner";
-import { HttpChecker } from "../../../src/server/checker/runner/http/runner";
+import { CommandChecker } from "../../../src/server/checker/runner/command/execute";
+import { HttpChecker } from "../../../src/server/checker/runner/http/execute";
 import { ProbeStore } from "../../../src/server/checker/store";
 import { rmRetry } from "../../helpers";
 
@@ -22,7 +24,7 @@ beforeAll(() => {
   ensureRegistered();
 });
 
-const httpTarget: ResolvedTarget = {
+const httpTarget: ResolvedHttpTarget = {
   expect: { maxDurationMs: 3000, status: [200] },
   group: "default",
   http: {
@@ -39,7 +41,7 @@ const httpTarget: ResolvedTarget = {
   type: "http",
 };
 
-const commandTarget: ResolvedTarget = {
+const commandTarget: ResolvedCommandTarget = {
   command: {
     args: ["-c", "1", "localhost"],
     cwd: "/tmp",
@@ -119,7 +121,7 @@ describe("ProbeStore", () => {
   });
 
   test("同步更新已有 target", () => {
-    const updated: ResolvedTarget = {
+    const updated: ResolvedHttpTarget = {
       ...httpTarget,
       http: { ...httpTarget.http, url: "https://example.com/v2" },
     };
@@ -287,7 +289,7 @@ describe("ProbeStore", () => {
 
   test("删除 target 级联删除 check_results", () => {
     const cascadeStore = new ProbeStore(join(tempDir, "cascade.db"));
-    const cascadeTarget: ResolvedTarget = {
+    const cascadeTarget: ResolvedHttpTarget = {
       group: "default",
       http: {
         headers: {},