normalizeOpencodeGoModel was prefixing model IDs with 'opencode-go/'
(e.g. 'opencode-go/deepseek-v4-flash'), but the Pi SDK sends the model
id field as the model name in API requests. The OpenCode API expects
bare model names (e.g. 'deepseek-v4-flash'), not prefixed ones.
Now strips the 'opencode/' or 'opencode-go/' prefix entirely so the
registered model ID matches what the API expects.