dsh-subagent-setting: Unified configuration of subagent default model in web settings page

## Introduction If you run multi-subagent task flows in DeepSeek Harness (DSH), you might encounter a specific issue: the default configurations for a subagent's provider, model, and reasoning effort are scattered across configuration files. To switch to a different default model, you must manually edit the YAML file and then verify that every subagent has adopted the new values. `dsh-subagent-setting` brings this to a web settings page: select the default provider, model, and reasoning effort on the interface, and newly created subagents will take effect immediately. You can also choose to apply this to existing subagents.