Add a show or read flag to the /plan command

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

During a session, the generated plan may be updated, or users may simply want to review it again later. Currently, there is no easy way to access the active plan except by manually searching through the Copilot session files using the session ID.

### Proposed solution

Adding a flag such as /plan show or /plan read would make it much easier to view the latest version of the current session’s plan.

### Example prompts or workflows

/plan show

or

/plan read

### Additional context

_No response_