[SECURITY] Add path traversal protection to prevent malicious file access

[jonasyr]

Closes #10

Problem

The codebase-to-text tool was vulnerable to directory traversal attacks, allowing malicious repositories to access files outside the intended directory using paths like ../../../etc/passwd. This could expose sensitive system files when processing untrusted repositories.

Solution

• Added _validate_file_path() method that uses os.path.commonpath() to ensure all file paths stay within the base directory
• Integrated path validation into both file processing (_process_single_file) and folder parsing (_generate_file_entries) pipelines
• Added comprehensive security logging for rejected paths when verbose mode is enabled
• Handles symlinks by resolving them with os.path.realpath() before validation

Tests

• 15 new security-focused test cases covering:
  • Legitimate files (should pass)
  • Directory traversal attempts with ../ patterns (should be blocked)
  • Absolute paths outside base directory (should be blocked)
  • Malicious symlinks pointing outside (should be blocked)
  • Internal symlinks (should pass)
  • Security verbose logging validation
  • Performance impact verification (<5% overhead)
  • Error handling for edge cases

Coverage/CI Notes

• All existing tests pass (no regression)
• Cross-platform testing for Windows/Unix path handling

Risks

• Low risk: Purely additive security feature with no API changes
• Backward compatible: No breaking changes to existing functionality
• Performance: Minimal overhead (~2-3%) from path validation operations

Rollback Plan

If issues arise:

1. Revert the three modified methods to original versions
2. Remove _validate_file_path() method
3. Remove security test classes
4. All functionality returns to pre-patch behavior

Testing Checklist

• Unit tests for path validation logic
• Integration tests with malicious path simulation
• Performance impact verification
• Cross-platform path handling (Windows/Unix)
• Symlink handling on supported systems
• Verbose logging verification
• Existing functionality regression testing
• Error handling for edge cases