Add providers doc

docs/providers.md

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+# Providers (OpenCode V2)
+
+OpenCode includes a provider and model catalog from models.dev. For most providers, connect an account first, then select a model.
+
+## Setup
+Run /connect, choose a provider, and enter its credentials. Then run /models to select one of its models.
+
+## Custom
+Add a provider when its API is not already in the catalog. This OpenAI-compatible example defines the credential, runtime package, endpoint, and first model together.
+
+Example:
+{
+  "$schema": "https://opencode.ai/config.json",
+  "providers": {
+    "acme": {
+      "name": "Acme",
+      "env": ["ACME_API_KEY"],
+      "package": "@opencode/ai/providers/openai-compatible",
+      "settings": { "baseURL": "https://llm.acme.example/v1" },
+      "models": { "qwen3-coder": { "name": "Qwen 3 Coder" } }
+    }
+  }
+}
+
+The providers object is keyed by the provider ID used in model references, such as acme/qwen3-coder.
+
+Fields: name, env, package, canonical, compatibility, settings, headers, body, models.
+
+## Endpoint
+Override settings.baseURL to send a catalog provider through a proxy or compatible endpoint.
+
+## Packages
+Native package options include openai, openai-compatible, anthropic, google, azure, amazon-bedrock, openrouter, xai, etc. You can also use npm packages or file:// URLs.
+
+## Errors
+- No model is available for session: no enabled model selectable.
+- Model unavailable: provider/model: provider inactive, model ID absent/disabled, or discovery no longer returns it.
+- Cannot initialize provider/model: a placeholder remains in baseURL. Set that variable on the OpenCode server or replace the template with a complete endpoint.