Research and Implement: OpenCode adapter

[PeterShin23]

A user mentioned that they don't use claude code, gemini, or codex, but they would like to use this for what they do use, one of which is OpenCode.

Research whether OpenCode supports such functionality without a hacky solution that may break ToS. Until this criteria is met, keep this issue open.

Note: Writing an adapter specifically for CLI is not critical. SDK/HTTP API approach is fine as long as it's well documented and works well with existing design.

[PeterShin23]

Opencode supports Claude Code oauth it seems like as well as openrouter. These two will be the primary way we integrate these