dsh-memory-canvas: DSH Plugin: Unloading Tool Logs to refs and Maintaining the Mermaid Canvas

The `dsh-memory-canvas` plugin is designed to address the problem of tool logs overflowing the context window when DeepSeek Harness (DSH) processes complex tasks. It offloads large volumes of logs to the local file system while keeping the Mermaid task canvas in context and retrieving memories by `node_id`. The plugin provides three tools—`memory_offload`, `memory_recall`, and `memory_canvas`—supports custom storage paths via environment variables, and requires no background daemon, making it suitable for DSH agent tasks that must manage large amounts of tool output.