Generate a market requirements document (MRD) from a product idea and save it as Markdown under the current project's markdown/ directory. Use when the user asks to write, draft, create, produce, or refine an MRD, market requirements document, product opportunity document, market demand analysis, product idea validation document, or asks for "写 MRD", "市场需求文档", "产品立项文档", "产品机会分析", especially when input is only an idea and the output should be structured, visual, and decision-oriented.