feat: add duplicate code detection

[Azaucifer]

Description

Adds structural duplicate-function detection to the codebase health analyzer using Python AST normalization.

Changes

• Detects structurally identical functions across Python files
• Normalizes function names and arguments before comparison
• Skips files containing syntax errors
• Displays duplicate function groups in the terminal report
• Includes duplicate function groups in JSON reports
• Adds unit and integration tests for duplicate detection

Testing

• 61 tests passing
• Duplicate functions tested
• Non-duplicate functions tested
• Syntax-error handling tested
• JSON report integration tested

Example

The analyzer can identify functions such as:

• add(a, b)
• calculate(x, y)

as duplicates when their underlying structure is identical.