Auto-rename session on exit

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

When closing Copilot CLI, sessions are left with default/unnamed labels unless the user manually runs /rename before exiting. This makes it hard to find and resume past sessions later, as they lack meaningful descriptions. Most users forget to rename, resulting in a list of unnamed sessions that all look the same.

### Proposed solution

Add a configuration option (e.g., auto_rename_on_exit: true) that automatically triggers the /rename auto-generation
logic when the user exits the CLI via /exit, /quit, ctrl+d, or ctrl+c ×2. This would use the existing conversation-based name
generation — no user input required.

### Example prompts or workflows

_No response_

### Additional context

_No response_