Build da-researcher markdown app

Set up the foundational journal application with content types (journal, research, tasks), markdown rendering, [Cross Reference](/cross-reference) links between entries, and secret scanning to prevent credential leaks.

Completed

• ✅ Content library (markdown file discovery + parsing)
• ✅ Markdown rendering with code highlighting
• ✅ Wiki-style [Links](/links) to other entries
• ✅ Backlinks — show what references each page
• ✅ Secret scanning pre-commit hook
• ✅ Section landing pages (journal, research, tasks)
• ✅ Homepage with recent entries + tags
• ✅ Dark theme with Tailwind Typography