PR screenshots

Upload screenshots and videos with the GitHub CLI's --attach, which needs gh 2.99 or later (gh --version). It works like dragging a file into the PR on github.com: the file goes into GitHub's attachment storage and shows inline, with videos as a player. It uses the GitHub CLI's login, so there are no keys to set up.

Attachments follow the repository's visibility, so in a private repository they stay private. In a public repository anyone can see them, so only attach screenshots of seeded or test data there, never real personal data, secrets, tokens or credentials.

Steps

1. Take screenshots at the sizes that matter, such as desktop and mobile, with descriptive file names.
2. Write the section in the body file, referencing each local file under a line on what it shows. Write the alt text in the reference: