[FEATURE] Different models across instances

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Using Haiku in one terminal instance applies it to other instances (Claude MAX)

Proposed Solution

Session-specific model settings.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

API and model interactions

Use Case Example

_No response_

Additional Context

_No response_