deepseek-harness-flowchart: Add a themed Mermaid flowchart renderer for DeepSeek Harness

# Introduction In DeepSeek Harness (DSH), there are two common approaches to enabling an agent to "draw a flowchart": the first is having the model output Mermaid source code, which the user then pastes into a renderer to generate the image; the second is writing a script tool that directly manipulates the file system, allowing the model to generate and save files locally. The former breaks the flow outside of the conversation, while the latter bypasses DSH's file writing approval and sandbox policies. deepseek-harness-flowchart takes a different approach: it mounts a `render_flowchart` tool for DSH to render Mermaid flowcharts locally into themed, self-contained SVGs, with file writing delegated back to Harness.