refactor: 移除顶层 defaults 配置段，简化为 target 显式字段 > 代码内置默认值

- 移除 DefaultsConfig 类型、ProbeConfig.defaults 字段
- 移除 CheckerSchemas.defaults、ResolveContext.defaults、CheckerValidationInput.defaults
- 更新所有 checker schema/resolve/validate 删除 defaults 合并逻辑
- 更新 config-loader 不再读取传递 defaults
- 更新测试、README、DEVELOPMENT、probes.example.yaml
- 重新生成 probe-config.schema.json（不含 defaults）
- 同步 delta specs 到主规范
- 归档 openspec change

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

@@ -209,12 +209,11 @@ export class CommandChecker implements CheckerDefinition<ResolvedCommandTarget>
 
   resolve(target: RawTargetConfig, context: ResolveContext): ResolvedCommandTarget {
     const t = target as RawTargetConfig & { cmd: CommandTargetConfig; type: "cmd" };
-    const cmdDefaults = context.defaults["cmd"] as undefined | { cwd?: string; maxOutputBytes?: string };
 
-    const cwd = t.cmd.cwd ?? cmdDefaults?.cwd ?? ".";
+    const cwd = t.cmd.cwd ?? ".";
     const resolvedCwd = resolve(context.configDir, cwd);
 
-    const maxOutputBytes = parseSize(t.cmd.maxOutputBytes ?? cmdDefaults?.maxOutputBytes ?? "100MB");
+    const maxOutputBytes = parseSize(t.cmd.maxOutputBytes ?? "100MB");
 
     const env = { ...process.env, ...(t.cmd.env ?? {}) } as Record<string, string>;