ACP: session/new response reports incorrect currentModelId

### Describe the bug

When starting a new ACP session, the session/new response's models.currentModelId and configOptions (category: model) both report claude-sonnet-4.6 regardless of the actual model being used for inference.

### Affected version

1.0.5

### Steps to reproduce the behavior

1. Start copilot --acp --stdio (or via a client that connects over ACP)
2. Send initialize then session/new
3. Observe models.currentModelId in the response - it reports claude-sonnet-4.6
4. Send a session/prompt asking the model to identify itself
5. The model responds that it is claude-opus-4.6 (or whatever the configured model actually is)

### Expected behavior

currentModelId should reflect the actual model that will handle prompts. If the model changes after session creation (e.g. via --model flag or internal routing), a config_option_update notification with category: "model" should be sent to inform the client.

### Additional context

_No response_