dsh-custom-prompt: Injecting Custom Rules into DSH's Global System Prompt

`dsh-custom-prompt` is a DeepSeek Harness plugin designed to address the need for unified management of global behavior constraints. It registers user-defined text as part of the system prompt (order 50) and applies changes in real time via the settings page, without requiring a restart. The plugin supports variable usage, reset functionality, and state control, but note that the browser connects via loopback and variable support is strict.