chore(gator): default to GPT-6.1 Sol - #4065

Summary

Use GPT-6.1 Sol with medium reasoning for new Gator launches by default.

Related Issue

Closes #4064

Changes

• Change the Codex model in scripts/agents/gator/agent.yaml to gpt-6.1-sol.
• Preserve medium reasoning, model overrides, and existing sandboxes.
• Review the Gator launcher skill; its model-override examples remain valid and need no change.

Testing

• Checks appropriate to the affected code and behavior pass: Ruby YAML parsing and assertions for model/reasoning; git diff --check.
• Unit/E2E tests: not applicable to this manifest-only change; no live model launch performed.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs: not applicable.