fix: route OpenCode via Copilot, not OpenAI

Remove OPENAI_API_KEY from workflow env so the api-proxy's port 10004
falls through to the Copilot route (COPILOT_GITHUB_TOKEN) instead of
routing to api.openai.com with an invalid key.

The api-proxy resolveOpenCodeRoute priority is:
  OPENAI_API_KEY > ANTHROPIC_API_KEY > Copilot token

With OPENAI_API_KEY removed, it uses the Copilot token to route
to api.githubcopilot.com (already in allowed domains).

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

Author: lpcox