[gbrain] ZeroEntropy has been acquired by Notion

gbrain uses a ZeroEntropy API key by default. It looks like that API has been sunsetted and new signups have been disabled.

From their [blog post](https://zeroentropy.dev/articles/zeroentropy-is-joining-notion):

> ZeroEntropy has been acquired by Notion. The whole team is joining to keep making Notion AI more useful and efficient.
All of our models are now fully open-source under Apache 2.0.
All ZeroEntropy products remain fully supported until September 4th, 2026, after which they will be sunset. New signups are disabled.
Existing customers should follow our [migration guide](https://zeroentropy.dev/articles/zeroentropy-migration-guide).

The embedding model is only 4B and can now be self hosted (e.g. using oMLX on Mac).

https://huggingface.co/zeroentropy/zembed-1-embedding

The catch: the existing zeroentropyai recipe (~/gbrain/src/core/ai/recipes/zeroentropyai.ts) isn't a generic "call this model" wrapper — it's wired to ZeroEntropy's specific non-standard API shape. gbrain's gateway has a dedicated zeroEntropyCompatFetch shim (gateway.ts:987) that rewrites the request path to /models/embed and reshapes the response from `{results:[{embedding}]}` into the OpenAI-standard `{data:[{embedding,index}]}` the AI SDK expects. That shim only fires for recipe.id === 'zeroentropyai'.

Please improve support for ZeroEntropy from different providers.

Read ~/gbrain/src/core/ai/recipes/zeroentropyai.ts and gateway.ts around line 987 first. Trace how the recipe-specific request and response handling reaches the AI SDK, then identify how support can work with ZeroEntropy from different providers. Done means the integration is no longer tied only to the existing zeroEntropyai-specific shim.

Written by the indexing model from the issue text.