[FEATURE] /model command in the SDK

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

While /compact is now handled via the SDK, other commands like /model are not. This is probably the most critical command to expose in order to allow seamlessly controlling the model via ACP integrations (see #6686).

Proposed Solution

Just like /compact is available via the SDK, expose /model. Essentially, we would also need available models if possible.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Developer tools/SDK

Use Case Example

In Agent Client Protocol implementations like zed-industries/claude-code-acp this would enable more built-in commands to work.

Additional Context

_No response_