Added assist mermaid export to render the fenced mermaid blocks inside a markdown file out to SVGs. With a file argument it pulls every ```mermaid block and writes them next to each other as <stem>-<index>.svg; with no argument it scans *.md in the current directory. --out <dir> overrides the destination and --index <n> renders just the nth block, which is handy when iterating on a single diagram. The actual rendering goes through Kroki — POST the source, get back SVG — and the endpoint is configurable via mermaid.krokiUrl for anyone running a self-hosted instance.