Add Graphite workflow Cursor command (#88939)
### What?
Adds a Cursor command file (`.cursor/commands/gt-workflow.md`) that
provides guidance on using Graphite (gt) instead of git for branch and
commit operations.
### Why?
To make the Graphite workflow easily accessible within Cursor IDE via
the `/gt-workflow` command, helping contributors follow the repository's
git workflow conventions.
### How?
Creates a new Markdown command file that includes:
- Quick reference table of Graphite commands
- Safety rules for stack operations
- Multi-branch fix workflow example
- Checklist for verification
