docs: explain OpenCode GitHub Copilot authentication (#26149)

Explain how to use GitHub Copilot models in OpenCode with the stored
GitHub credential, without a separate device login. Link to the existing
credential setup rather than repeat it.

Verified against the OpenCode kit startup hook and its credential tests
in the release source. Scoped rumdl and Vale checks pass; the two PRs
merge cleanly. Live Copilot authentication wasn't tested.

@netlify /ai/sandboxes/agents/opencode/

Generated by Codex

content/manuals/ai/sandboxes/agents/opencode.md

@@ -51,6 +51,14 @@ $ sbx secret set openrouter
 You only need to configure the providers you want to use. OpenCode detects
 available credentials and offers those providers in the TUI.
 
+### GitHub Copilot
+
+To use GitHub Copilot models in OpenCode, configure a
+[GitHub credential](../configuration/credentials.md#github-token) for an
+account with Copilot access. When OpenCode starts, Docker Sandboxes configures
+its GitHub Copilot provider using that credential. You don't need a separate
+device login inside OpenCode.
+
 ### OpenCode Zen API keys
 
 OpenCode Zen API keys aren't part of the built-in OpenCode credentials that