feat(opencode-go): add the seven models upstream serves but the registry omits (#4357)

Upstream /zen/go/v1/models serves 35 ids; the registry listed 28. Add the seven
missing models with their corresponding supported and target formats:
- deepseek-v4.1-flash
- mimo-v2.6-flash, mimo-v2.6-pro
- space-bunny-free
- omen-alpha
- grok-4.7 (responses-only)
- gpt-6-luna (responses-only)