Adds 'vercel' as a recognized provider throughout the model resolution
system, enabling users with Vercel AI Gateway to use it as a universal
fallback for OpenAI, Anthropic, and Google models.
The gateway transform infers the sub-provider from canonical model names
(claude->anthropic, gpt->openai, gemini->google) and delegates to the
appropriate provider-specific transform, producing model strings like
vercel/anthropic/claude-opus-4.6.
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>
Align runtime defaults, tests, docs, and generated artifacts with the newer GPT-5.4 baseline. Keep think-mode and prompt-routing expectations consistent after the model version bump.
YeonGyu-Kim