feat: add Agnes AI as default provider, support multi-provider switching

- Default provider changed from Zhipu to Agnes AI (agnes-2.0-flash)
- Add --provider flag: agnes (default), zhipu, custom
- Add AGNES_API_KEY env var support (still compatible with ZHIPU_API_KEY)
- Add provider presets system for easy switching between AI backends
- Add .gitignore to prevent config.json with API keys from being committed
- Update usage examples

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -7,3 +7,4 @@ config.json
 
 # Temp files
 doc_update.json
+config.json