/btw slash command not available in VS Code extension

Summary

The /btw slash command works in the Claude Code CLI but is not
available when using Claude Code via the VS Code extension.

Expected

/btw <question> should fork a single-turn side query from the
current conversation context, the same way it does in the terminal CLI.

Actual

/btw is not recognized as a slash command in the VS Code extension.

Why this matters

/btw is useful for asking quick side questions without polluting the
main conversation history (and avoiding re-billing the full transcript
on each follow-up). VS Code extension users currently have to drop to
the integrated terminal or start a separate session to get the same
behavior.

Request

Port /btw to the VS Code extension to bring it to parity with the CLI.