Add a method to /exit from Copilot CLI regardless of nested sessions

Describe the feature or problem you'd like to solve

Add a method to /exit from Copilot CLI regardless of nested sessions

Proposed solution

Add a /exit behavior that gracefully terminates the current Copilot CLI, either by changing the /exit behavior, adding a subcommand or adding a separate command.

Example prompts or workflows

N/A

Additional context

When using a Copilot CLI session, and running several /new sessions after completing unrelated work, invoking the /exit command will currently exit the current session and show the previous one - instead of existing Copilot CLI.

This matches behavior matches with the current docs:

/exit, /quit: Close the current session. If other sessions are running, this foregrounds the newest remaining one instead of quitting. Quits the CLI only when it is the last open session. /exit print always tears down the CLI and offers to dump the transcript.

To fully close Copilot CLI after multiple /new turns on either has to run /exit multiple times or terminate the process.

Start by tracing the existing /exit command and the session handling used after multiple /new sessions. Reproduce the documented behavior with nested sessions, then verify that the chosen /exit behavior closes the entire Copilot CLI while preserving the existing current-session behavior where appropriate.