dsh-custom-instructions: DeepSeek Harness Global Instruction Management

`dsh-custom-instructions` is a lightweight plugin designed for DeepSeek Harness (DSH) to provide unified management of global instructions through a graphical interface. It allows users to edit content in the “Custom Instructions” section of the settings panel. The file is automatically saved to `~/.dsh/AGENTS.md` and, through DSH’s built-in mechanism, applies to all sessions and models on the current machine. It is easy to install, requires no build, supports live preview and instant application, making it suitable for scenarios that require consistent behavioral rules across multiple sessions.