feat: improve model configuration and settings management (#59)

* feat: improve model configuration and settings management

- Refactor model configuration system with unified settings approach
- Add support for user-level default models and project-level current models
- Implement automatic saving of API key and base URL from command line arguments
- Remove model descriptions for cleaner UI and easier extensibility
- Fix commander.js subcommand argument parsing with fallback mechanism
- Improve configuration file structure and priority handling
- Add comprehensive model configuration utilities in src/utils/model-config.ts

Configuration priority: project current model > user default model > system default
Both main command and git subcommand now save credentials automatically

* refactor: unify settings management

- Add SettingsManager singleton for user/project settings
- Eliminate duplicate settings code across files
- Clear separation: user settings vs project settings

* docs: improve README clarity and organization

- Simplify setup section with quick start approach
- Consolidate configuration info into unified section
- Add clear explanation of user vs project settings
- Document OpenAI-compatible API support
- Remove duplicate model configuration sections

* bump sdk

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Co-authored-by: Ismail Pelaseyed <homanp@gmail.com>