Add MegaNova AI provider extension following the plugin architecture pattern.
MegaNova provides access to 30+ models through an OpenAI-compatible API,
including reasoning models (GLM-5, DeepSeek-R1, Kimi K2) and open-source
models (Llama, Qwen, MiniMax, Mistral).
- Create extensions/meganova/ with plugin manifest, onboard config, and
catalog builder
- Register in bundled provider plugins and contract registry
- Export model definitions via plugin-sdk for extension use
- Add provider docs and navigation entry
- Add meganova-api-key auth choice and CLI flag
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