DSH Tool Markdown: DSH Markdown Tool Plugin

`omdsh-dev/dsh-tool-markdown` is an MIT-licensed document processing plugin for DSH, designed to address format instability and security risks in HTML, Markdown, and table conversions. The plugin registers a `markdown` tool and supports four core functionalities through different `action` values: converting HTML to GFM Markdown, converting Markdown to whitelist-safe HTML, normalizing HTML or pipe-delimited text into GFM tables, and generating nested table of contents based on headings. The plugin adopts a zero-dependency, pure-function architecture with strict security boundaries: it prohibits script execution, blocks remote resource loading, and automatically strips dangerous tags such as `script` and `style`; output only allows whitelisted tags, forces text escaping, and retains only links.