dsh-widthkeeper: Adding a controllable "width keeper" to the DeepSeek Harness session content column.

Introducing dsh-widthkeeper, a DSH client plugin maintained by YuLaiZ. It addresses issues such as the narrow hot zone of the built-in width handle, default width changing based on column width via formulas, and difficulty in precise width adjustment. It provides a slider below the input box, precise pixel input, automatic reading, collapsible controls, and settings page partitioning. It sets the `--dsh-chat-content-width` variable and hides the native handle in fixed mode. It operates only on the main session, with preferences stored in `localStorage`. It supports standalone installation for both Web and Desktop versions via GitHub or npm, is compatible with DSH >=0.2.0-rc.1, is licensed under MIT, and has a codebase based on Jinngoo/dsh-conversation-width.