Docs(opencode): add API key provider config example

[smallgun01]

What problem does this solve?

The existing docs/opencode.md examples (Ollama Cloud, xAI Grok) only cover OAuth-based providers. API-key-based providers like opencode-go require a different config structure — the full provider.options.apiKey block — which is not documented anywhere. Users hit this gap immediately when trying to set up API-key providers.

Closes #1200

At a Glance

N/A — docs-only change.

Prior Art & Industry Research

Not applicable — docs-only change, adding a missing config example.

Proposed Solution

Add a new ## Example: API Key Providers section to docs/opencode.md, placed after the existing xAI Grok example and before ## Notes. The example uses opencode-go as a representative API-key provider, showing:

• The full provider.options.apiKey + models block
• ${OPENCODE_API_KEY} env var expansion
• Restart step to pick up config

Why this approach?

Follows the exact same structure and conventions as the existing two provider examples (authenticate → set config → restart → verify). Uses kubectl exec + heredoc format consistent with the rest of the doc. Minimal, focused — one realistic example covers the pattern for all API-key providers.

Alternatives Considered

• Adding a generic "Provider Config Reference" section: too much scope for a single example gap. A focused, copy-pasteable example is more useful to users who already understand the OAuth examples.
• Including kubectl create secret step: unnecessary noise — Kubernetes Secret creation is covered elsewhere and varies by deployment.

Validation

• Links are valid (all internal links point to existing anchors or docs)
• Renders correctly in GitHub preview (standard markdown, code block)
• kubectl exec heredoc syntax matches existing examples (unquoted << EOF allows env var expansion)
• JSON schema matches the opencode.json spec as documented at opencode.ai
• Discord Discussion URL: https://discord.com/channels/1491295327620169908/1520318436507648130

[thepagent]

should it be {env:VAR} format?

https://opencode.ai/docs/config/#env-vars

[smallgun01]

You're right — {env:OPENCODE_API_KEY} is the official portable syntax per opencode docs.
Updating the example.

Side note for anyone reading: the ${OPENCODE_API_KEY} form in a kubectl exec heredoc also
works because the shell expands it before writing the file — but only in shell-generated configs.
For users hand-writing opencode.json or running via Docker CMD / systemd, {env:VAR} is the
only correct option.

Thanks @thepagent.