feat: add config subcommand to manage API keys with hot-reload

- Add config add: Add or update API key for a provider
- Add config list: List all configured API keys (with masking)
- Add config remove: Remove API key for a provider
- Support for all LLM providers (dashscope, deepseek, openai, etc.)
- Hot-reload: Updates take effect immediately without restart
- User-friendly provider name mapping
- Both host config file (default.cfg) and container config (ai-proxy.internal.yaml) are updated
- Touch config file to trigger hot-reload
- Updated help documentation with examples

Author: johnlanni