feat: 希望在Pi引入opencode go 套餐的支持

### 使用场景 / Use case

期望：Pi支持使用opencode go 套餐

### 当前问题 / Current limitation

1

### 期望方案 / Proposed solution

_No response_

### 已考虑的替代方案 / Alternatives considered

_No response_

No files, tests, or entry points are named. Start by locating Pi's provider configuration and existing plan integrations, then determine how the opencode go subscription should be represented. Done means Pi can use that plan and the integration is covered by appropriate tests.

Written by the indexing model from the issue text.