Note in the UI when a user is working in a git worktree

## Summary

The CLI does not indicate when Copilot is working in a git worktree. There is no visible signal in the UI to distinguish a worktree from a standard checkout.

## Motivation

A worktree is not distinguishable from a standard checkout in the UI, so the working context is ambiguous when Copilot operates in a worktree.

## Proposal

Surface worktree context in the CLI, similar to how the current branch is displayed, when Copilot is working in a worktree.

## Example prompts or workflows

_No response_

## Additional context

_No response_