feat(opencode): add OpenAI provider support

[martinffx]

Summary

Adds OpenAI as a selectable OpenCode provider with direct model defaults and provider validation.

Changes

• Add OpenAI provider choices, models, and agent defaults.
• Accept OpenAI in configuration schemas and prompts.
• Show the documented OpenCode CLI authentication command after OpenAI initialization.
• Cover provider defaults, config validation, and onboarding output.

Testing

• bun test
• bun run typecheck

Checklist

• Tests pass
• Type-check passes
• Conventional commit included