refactor: replace inline markdown with markdown package dependency
Remove 606 lines of hand-rolled markdown parsing from main.lux and the unused markdown.lux split file. Replace with `import markdown` using the new markdown package (path dependency at ../markdown). This fixes the heading-in-list rendering bug where `- ### Title` was showing literal `### ` text. Now renders as `<li><h3>Title</h3></li>`. Also adds strikethrough support (~~text~~). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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