feat(llm): add native Novita AI provider via OpenAI-compatible endpoint (#155)

* feat(llm): add native Novita AI provider via OpenAI-compatible endpoint

Wire Novita AI (https://api.novita.ai/openai/v1) as a first-class LLM
provider, following the same pattern used for DeepSeek/HuggingFace/NanoGPT:
dispatched through the existing OpenAIAdapter, resolved via NOVITA_API_KEY,
with a default model, model catalog entries, config/setup wiring, and
regression tests.

* fixup(llm): use deepseek/deepseek-v4-flash-0731 as the second Novita model

Swap the curated Novita model catalog's second entry from
qwen/qwen3-coder-30b-a3b-instruct to deepseek/deepseek-v4-flash-0731 per
requested model selection. Verified live against api.novita.ai/openai/v1/models.

* fixup(llm): replace superseded DeepSeek Flash id with current Pro flagship

QA flagged that deepseek/deepseek-v4-flash-0731 has been superseded in the
live Novita catalog; deepseek/deepseek-v4-pro-0813 is now the current
serving DeepSeek flagship. Swap the second Novita model catalog entry
accordingly.

---------

Co-authored-by: jax-novita <jax-novita@users.noreply.github.com>
Co-authored-by: Joseph Magly <1159087+jmagly@users.noreply.github.com>

.env.example

@@ -10,6 +10,8 @@ DEEPSEEK_API_KEY=sk-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 # Hugging Face — open models via the OpenAI-compatible Inference Providers router.
 # HF_TOKEN is canonical; HUGGINGFACE_API_KEY / HUGGINGFACE_TOKEN also work.
 HF_TOKEN=hf_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+# Novita AI — OpenAI-compatible API serving open-source and frontier models.
+NOVITA_API_KEY=sk_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 GROQ_API_KEY=
 TOGETHER_API_KEY=
 REPLICATE_API_TOKEN=