feat: add Novita AI as optional LLM provider

[Alex-wuhu]

This PR adds Novita AI as an optional LLM provider.

Changes

• Added Novita provider to PROVIDERS array in providers.ts
• Added Novita model factory in llm.ts using ChatOpenAI with Novita's OpenAI-compatible endpoint
• Updated env.example with NOVITA_API_KEY
• Created src/providers/novita-provider.ts with configuration constants
• Updated Novita recommended model presets:
  • novita:deepseek/deepseek-v3.2 (fast model)
  • novita:minimax/minimax-m2.5
  • novita:zai-org/glm-5

Notes

• Endpoint follows Novita OpenAI-compatible base URL: https://api.novita.ai/openai
• Existing provider behavior remains backward compatible