Default costume agents

### Describe the feature or problem you'd like to solve

It's really annoying constantly changing to my default agent every time I /clear or /new.

### Proposed solution

Being able to select the default agent we want to run new sessions.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by tracing how the /clear and /new commands initialize sessions and how the current agent is selected. Review any existing configuration or agent-selection entry points, then verify that a user-selected default agent is used consistently for new sessions and after clearing one.

Written by the indexing model from the issue text.