GitHub stacked PRs: gh pr edit --base fails even when the base is unchanged

When re-opening and updating PRs that belong to a GitHub stack (each PR's base is the branch below it), gh pr edit N --title T --base B failed with 'GraphQL: Cannot change the base branch because the pull request is part of a stack', even though B was already the PR's base. Pass only the fields you're actually changing (title, body). Also make PR-opening scripts idempotent (reuse an existing open PR for a head branch, and re-open closed ones instead of recreating them), because gh hits transient 'error connecting to api.github.com' failures and a rerun has to pick up where it stopped. Create placeholder bodies first, then fill in cross-PR numbers once every PR exists.