Native worktree support

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

Need native support for worktrees

### Proposed solution

With copilot cli doing most of the coding tasks . Its essential that its able to work on multiple tasks on the same time without interfering with each other and keep the main clone available for the actual user to explore, which is what worktrees are for. So a native support for worktrees would be of immense help in boosting productivity.

### Example prompts or workflows

copilot --worktree

### Additional context

_No response_

The issue names no files, tests, or implementation entry points. Start by locating the CLI option parsing and the existing Git integration, then compare the proposed `copilot --worktree` workflow with the project's current task-isolation behavior; done would require documented native worktree support without interfering with the main clone.

Written by the indexing model from the issue text.