Allow /fork to open a new terminal and add copilot --fork #4580

Describe the feature or problem you'd like to solve

The interactive /fork command moves the current terminal into the fork, and Copilot CLI has no startup option for creating a fork from an existing session. This makes parallel terminal workflows unnecessarily manual.

Proposed solution

Support both of these entry points:

1. Add copilot --fork=<session-id> --name=<new-session-name>. It should create a new fork of the supplied session, apply the optional name to the new fork, and start the fork in the process that invoked the command. The source session name should remain unchanged.
2. Add an option such as /fork --new-terminal [name]. It should create the fork in a new terminal window or tab while the current terminal remains in the original session.

The command line option provides a portable building block when automatic terminal launching is unavailable. Users can combine it with their preferred terminal application.

If /fork --new-terminal cannot identify or launch the current terminal application, it should return the new session ID and a ready-to-run copilot --resume=<new-session-id> command instead of silently changing behavior.

Example prompts or workflows

1. Run /fork --new-terminal investigate-auth during a task. The fork opens separately, and the original session remains visible and interactive.
2. Run wt new-tab copilot --fork=<session-id> --name=investigate-auth to create and name a fork in a new Windows Terminal tab.
3. Run copilot --fork=<session-id> --name=alternative-design from another shell to create, name, and enter a fork of a specific session.
4. Run !wt new-tab copilot --fork=<session-id> --name=side-investigation inside Copilot CLI to launch the fork without abandoning the current session.

Additional context

This replaces #4578, which was submitted without the Feature Request form.

This is a focused follow-up to #2058. That issue requested the core /fork command and mentioned a new terminal window or tab in its proposed solution. The core command now exists, but terminal selection and process startup interfaces are still missing.

Related issue #1697 also describes working with forked sessions in separate terminals.

Confirmed with GitHub Copilot CLI 1.0.81-8: both copilot --fork=<session-id> and copilot --branch=<session-id> are rejected as unknown options.