Bug: JobConfig 手动设置 experiment_id 未同步到 environment

## 问题
`rock/sdk/job/config.py` 中 `JobConfig` 仅在模型初始化后的 validator 里同步一次 `experiment_id`。

当用户先初始化 `JobConfig` / `BashJobConfig` / `HarborJobConfig`，再手动执行 `cfg.experiment_id = "..."` 时，`cfg.environment.experiment_id` 保持旧值或 `None`。后续依赖 `config.environment` 创建 sandbox 时会拿到错误的 `experiment_id`。

## 预期
手动更新顶层 `experiment_id` 后，`environment.experiment_id` 保持一致。
替换 `environment` 对象时，已有的顶层 `experiment_id` 继续传导到新的 environment。

## 验收
- 增加单元测试覆盖初始化后赋值场景
- 增加单元测试覆盖替换 environment 场景
- 保持现有冲突日志语义

Start in rock/sdk/job/config.py by inspecting JobConfig and the BashJobConfig and HarborJobConfig variants, then find the existing unit tests for these configurations. Run those tests before adding coverage for updating experiment_id after initialization and replacing environment. Done means both synchronization paths work while preserving the existing conflict-log semantics.

Written by the indexing model from the issue text.