Add the ability to copy/clone another session

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

I have a session that has context in a place that I like, but I want to work with more than thing simultaneously with the same context.

### Proposed solution

Add a /clone or /copy to clone the context from another session. Essentially the same as /resume , but clones it as a new session.

Ideally /clone or /copy would also show the list of sessions if a sessionId isn't specified.

I tried to /resume a session to duplicate context, and messages from one affected the other. It was a fun experiment 🙃

In this instance, I wanted the main agent to review the current changes with a subagent, but I also wanted to create a skill for the new runbook the agent created. It broke both sessions until I killed one and then the copilot recovered.

### Example prompts or workflows

/clone
/clone (then select sessionId)

### Additional context

_No response_