fix: configure OpenCode with custom copilot-proxy provider

OpenCode doesn't auto-register providers from env vars alone - it needs
either /connect auth or explicit provider config. Configure a custom
'copilot-proxy' provider using @ai-sdk/openai-compatible pointing to
the AWF api-proxy at port 10004, and pre-populate auth.json with the
Copilot token.

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

Author: lpcox