fix: save optional config fields (lang, scope, type, ignore_patterns) - #9

Summary

Fix saveConfig to persist optional fields when using config --set.

Problem

When setting optional fields like lang, scope, type, or ignore_patterns, they were not being saved to the config file.

$ git-commit-ai config --set lang=en
✓ Config updated: lang (default_language) = "en"

# But the file didn't contain default_language!

Solution

Updated saveConfig to append optional fields when they are set:

• default_language
• default_scope
• default_type
• ignore_patterns

Test

$ git-commit-ai config --set lang=en
✓ Config updated: lang (default_language) = "en"

$ git-commit-ai config
Configuration:
  ...
  Default language: en
  ...

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