feat: add short aliases for config keys - #8

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

$ git-commit-ai config --list-keys
Valid config keys:
  backend
  model
  ollama_url (alias: url)
  openai_base_url
  temperature (alias: temp)
  default_scope (alias: scope)
  default_type (alias: type)
  default_language (alias: lang)

Short aliases:
  lang → default_language
  scope → default_scope
  type → default_type
  url → ollama_url
  temp → temperature