Add models doc

docs/models.md

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+# Models (OpenCode V2)
+
+Select model with /models. Use --model for one run. Set default model in config.
+
+Free models: set OPENCODE_DISABLE_FREE_MODELS=1 to hide free OpenCode models.
+
+Variants: add #variant, e.g. openai/gpt-6.1-sol#high. Define variants in model variants array.
+
+Compatibility: reasoningField, requireReasoning, maxTokensField, cacheFormat, etc.
+
+Local models: Ollama, LM Studio, vLLM auto-discovered. Custom OpenAI-compatible via providers config.
+
+References: provider/model with optional #variant. Provider and model IDs case-sensitive.