docs: improve Graphite workflow documentation (#89488)
### What?
Improves the Graphite workflow documentation in
`.cursor/commands/gt-workflow.md`.
### Why?
The previous documentation was unclear about when to use `gt` vs `git`
commands, leading to potential workflow issues.
### How?
- Clarifies that `gt` should be used for ALL git commands (not just
specific ones)
- Adds a critical section about staging changes BEFORE running `gt
create` to avoid creating empty branches
- Updates all `git` references to `gt` for consistency (`git show` → `gt
show`, `git status` → `gt status`, etc.)
- Improves the command reference table with `gt add`
- Reorganizes checklists for clarity
### Improving Documentation
- [x] Run `pnpm prettier-fix` to fix formatting issues before opening
the PR (handled by lint-staged)
