[Feature]: Integrate DeepSeek-V3 as an AI provider

Description

I propose integrating DeepSeek-V3 as a new AI provider in OpenCommit. DeepSeek-V3 is a powerful AI model that offers high performance and speed, making it a great alternative to existing providers like OpenAI, Anthropic, or Ollama.

This integration would allow users to use DeepSeek-V3 for generating commit messages, providing more options and flexibility in choosing AI models.

Suggested Solution

1. Add DeepSeek-V3 as a supported AI provider in the OpenCommit configuration.
2. Allow users to set DeepSeek-V3 as the default provider via the OCO_AI_PROVIDER config.
3. Support DeepSeek-V3's API endpoint and model options in the configuration.

Example configuration:

oco config set OCO_AI_PROVIDER=deepseek-v3
oco config set OCO_API_KEY=<DeepSeek_API_Key>
oco config set OCO_API_URL=<DeepSeek_API_URL>
oco config set OCO_MODEL=deepseek-v3

### Alternatives

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### Additional Context

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Start by locating the existing AI provider integrations and configuration handling in OpenCommit, then compare how provider, API URL, API key, and model settings are represented. Done means DeepSeek-V3 can be selected through OCO_AI_PROVIDER, its endpoint and model can be configured, and it generates commit messages through the documented settings.