refactor: 重命名 command checker 为 cmd checker 并适配跨平台测试

将 type/configKey 从 "command" 统一为 "cmd"，源码目录 runner/command/ → runner/cmd/，
spec 目录 command-checker/ → cmd-checker/，测试全部改用 bun -e 替代 Unix 系统命令，
归档 cmd-checker-enhancement 变更并同步 delta spec 到主 spec。

src/server/checker/runner/cmd/expect.ts

@@ -0,0 +1,19 @@
+import type { ExpectResult } from "../../expect/types";
+
+import { mismatchFailure } from "../../expect/failure";
+
+export function checkExitCode(exitCode: number, allowed: number[]): ExpectResult {
+  if (!allowed.includes(exitCode)) {
+    return {
+      failure: mismatchFailure(
+        "exitCode",
+        "exitCode",
+        allowed,
+        exitCode,
+        `exitCode ${exitCode} not in [${allowed.join(", ")}]`,
+      ),
+      matched: false,
+    };
+  }
+  return { failure: null, matched: true };
+}