feat(cli): Add diff command (compare workbooks)

[arcaputo3]

Summary

Add a command to compare two Excel workbooks and show differences.

Syntax

# Compare entire workbooks
xl diff old.xlsx new.xlsx

# Compare specific sheet
xl diff old.xlsx new.xlsx --sheet Sheet1

# Compare specific range
xl diff old.xlsx new.xlsx --sheet Sheet1 --range A1:F100

# Output format
xl diff old.xlsx new.xlsx --format json

Output Example

Comparing old.xlsx vs new.xlsx

Sheet: Sheet1
  Changed cells:
    A5: "Revenue" -> "Total Revenue"
    B5: 1000 -> 1500
    C5: =SUM(C2:C4) -> =SUM(C2:C4)*1.1
  
  Added cells:
    D5: "New Column"
  
  Deleted cells:
    E5: (was "Old Column")

Summary:
  - 3 cells changed
  - 1 cell added
  - 1 cell deleted

Comparison Types

• Values: Compare displayed values
• Formulas: Compare formula expressions
• Styles: Compare cell formatting
• Structure: Compare sheets, rows, columns

Implementation Notes

• Load both workbooks
• Iterate cells, compare values/formulas
• Track additions, deletions, changes
• Consider: ignore whitespace, case-insensitive

Acceptance Criteria

• Cell value comparison works
• Formula comparison works
• Added/deleted cells detected
• Cross-sheet differences shown
• JSON output format available

Priority

Low - Nice-to-have for auditing

Milestone

v0.8.0+