[FEATURE] Add /fork slash command support in VS Code extension

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single feature request

Problem Statement

In the CLI, users can type /fork to instantly fork the conversation at the current point. However, in the VS Code extension, the only way to fork is by hovering over a specific message to reveal the fork option. There is no way to quickly fork at the current point without mouse interaction.

Proposed Solution

Add /fork slash command support in the VS Code extension's chat input, matching the CLI behavior. This would allow users to type /fork to instantly create a new conversation branch from the current point.

Alternative Solutions

A "Fork" button in the VS Code extension's toolbar/action bar would also solve this.

Priority

Medium

Feature Category

IDE Integration

Use Case Example

During a long conversation, the user wants to try a different approach without losing the current conversation. In CLI, they simply type /fork. In VS Code, they should be able to do the same.

Additional Context

Related issues:

• #12629 (original session branching feature request for CLI)