Reject invalid --opencode-go values during non-TUI installs and detect existing OpenCode Go usage from the generated oh-my-opencode config so updates preserve the right defaults.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
YeonGyu-Kim