fix: configure OpenCode to use OpenAI provider via Copilot API proxy

OpenCode defaults to Google Gemini (gemini-3-pro-preview) which requires
GOOGLE_GENERATIVE_AI_API_KEY. The workflow only has COPILOT_GITHUB_TOKEN
routed through the api-proxy on port 10004 as OPENAI_API_KEY.

Set model to openai/gpt-4.1 in the opencode.jsonc config so OpenCode
uses the OpenAI provider, which routes through the Copilot API proxy.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox