dsh-chat-zone-std: DSH Standard Edition Chat Zone Plugin

`dsh-chat-zone-std` is a conversation zone management plugin for DeepSeek Harness (DSH), designed to address the organization of local conversation files. Built on the `dsh-std` ecosystem, it uses the mapping functionality of `@dsh-std/adapter-dsh` to provide commands for creating and listing conversations and conversation zones. The plugin stores conversation zones in a fixed directory structure of `~/dsh_CHAT/<local yyyy-mm-dd>/chatN`, and supports customizing the home directory via the `DSH_CHAT_ZONE_HOME` environment variable. Note that it is only responsible for creating folders, does not provide a chat interface, and depends on DSH Community v0.15.