Feature request: /rubber-duck slash command for independent critique in Copilot Chat

[digitarald]

Summary

Add a /rubber-duck slash command to Copilot Chat that provides an independent critique of the current approach — challenging assumptions, identifying blind spots, and suggesting alternatives before committing to implementation.

https://docs.github.com/en/copilot/concepts/agents/copilot-cli/rubber-duck

Implementation

In progress: #318662.