Implement Basic Markdown Rendering (markdown‑it)

[ZFordDev]

CoffeeMD will eventually unify multiple Markdown engines, but we need a simple baseline renderer to start with.

This issue adds basic markdown-it support so CoffeeMD can render Markdown → HTML.

Requirements

• Add markdown-it as a dependency
• Create src/parsers/markdownItParser.ts
• Export a simple function:

  export function renderMarkdown(input: string): string

• Integrate it into src/index.ts as the default renderer
• Add a simple test in tests/ to confirm rendering works

Notes

• No custom rules yet
• No SnapDock extensions yet
• This is just the first building block