[FEATURE]: DeepSeek: remove retired deepseek-chat / deepseek-reasoner from /models (models.dev#2979)

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Describe the enhancement you want to request

The /models list in opencode still shows four DeepSeek models:

• deepseek/deepseek-chat
• deepseek/deepseek-reasoner
• deepseek/deepseek-v4-flash
• deepseek/deepseek-v4-pro

DeepSeek discontinued deepseek-chat and deepseek-reasoner on 2026-07-24. They were never separate models — just aliases for deepseek-v4-flash modes (official changelog: https://api-docs.deepseek.com/updates). DeepSeek's official Models & Pricing page (https://api-docs.deepseek.com/quick_start/pricing) now lists only deepseek-v4-flash and deepseek-v4-pro, and the two stale entries return errors from the DeepSeek API.

Request: opencode should list only deepseek-v4-flash and deepseek-v4-pro under DeepSeek.

Since opencode's model list is fetched from the models.dev catalog (https://models.dev/api.json), the primary fix is a catalog change — removal requested at https://github.com/anomalyco/models.dev/issues/2979 (still open, unactioned). Until then, consider filtering or marking these entries deprecated client-side so /models stops surfacing dead aliases for every DeepSeek user.

Affected catalog files on dev:

• providers/deepseek/models/deepseek-chat.toml
• providers/deepseek/models/deepseek-reasoner.toml
• models/deepseek/deepseek-chat.toml
• models/deepseek/deepseek-reasoner.toml

Benefits: removes dead, erroring entries from the model picker and shows users only the two real DeepSeek models.