kordoc: compare_documents

Reproduce the compare_documents logic from src/mcp.ts with the public compare export.

Input Requirements

• Require exactly two target documents.
• Require the user to attach both documents or provide two concrete local paths.
• Accept only supported formats: .hwp, .hwpx, .pdf.
• Do not proceed if either document is missing.
• Preserve the user-provided ordering so source and target are not swapped implicitly.

Command