[Feature] Add PDF and DOCX export formats

[d-oit]

Description

The export engine currently supports Markdown, JSON, and HTML site export. PDF and DOCX export would make the knowledge base more shareable and professional.

Current Export Formats

• ✅ Markdown
• ✅ JSON
• ✅ HTML Site
• ❌ PDF
• ❌ DOCX
• ❌ CSV

Proposed Implementation

1. PDF Export: Use jsPDF or html2pdf.js for in-browser PDF generation
2. DOCX Export: Use docx npm package for Word document generation
3. CSV Export: Simple flat table export for spreadsheet import
4. Share export logic between browser and CLI (currently duplicated)

Acceptance Criteria

• PDF export available in both browser and CLI
• DOCX export available in both browser and CLI
• CSV export available
• Export logic shared (not duplicated) between browser and CLI
• All export formats tested

[github-actions]

👋 This looks like a duplicate of #199.

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