chore(gator): default to GPT-6.1 Sol with medium reasoning

[johntmyers]

User Story

I use OpenShell Gator to supervise PR reviews on my local docker-dev gateway in the gator workspace and want new launches to use GPT-6.1 Sol with medium reasoning by default.

Description

Change the default Gator Codex model from gpt-5.6-sol to gpt-6.1-sol, preserving the existing medium reasoning level and model override support.

Context

Per-launch overrides are available, but require repeating the desired model on each launch. This is a default configuration update, not a runtime or review-policy change. Existing sandboxes should remain unchanged.

Definition of Done

• Gator's manifest defaults to gpt-6.1-sol with medium reasoning.
• The manifest parses successfully and existing override behavior is unchanged.
• No existing Gator sandboxes are modified.