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.

首先跟踪现有的 /exit 命令，以及多个 /new 会话之后使用的会话处理方式。使用嵌套会话复现文档中描述的行为，然后验证所选择的 /exit 行为会关闭整个 Copilot CLI，并在适当情况下保留现有的当前会话行为。