dsh-composer-stretch: An Input Field Extension Plugin Built for DeepSeek Harness

`dsh-composer-stretch` is an input box extension plugin designed for DeepSeek Harness (DSH), aimed at addressing the limited space issue when editing long text and inserting attachments. Its core features include: automatically triggering the expand button when the input reaches three lines, as well as quickly expanding it by pressing Shift+Enter three times in a row; after expansion, the height approaches full screen and adapts to the conversation title and attachment components; in expanded mode, it supports sending messages with Cmd/Ctrl+Enter and automatically restores the default height after sending. In addition, it fixes the issue where scrolling after expansion causes the background conversation area to scroll as well. Installation in Web environments includes using the `dsh plugin add` command. The plugin runs entirely on the frontend and is compatible with DSH v0.2.