docs: add PR description guidance to AGENTS.md (#88408)
### What?
Adds documentation to AGENTS.md explaining how to create PRs with
descriptions when using Graphite.
### Why?
`gt submit --no-edit` creates PRs in draft mode without a description,
but all PRs require a description following the mandatory format. This
documents the recommended workflow for AI agents.
### How?
Added a new "Creating PRs with descriptions" section to the Git Workflow
documentation with example bash commands showing how to use `gh pr edit`
immediately after `gt submit --no-edit` to add PR descriptions.
