Update a branch

Run from a local work branch.

/update-branch
/update-branch release/1.x

When the current branch has one open pull request, always use that pull request's target branch, even if the user supplied another target. Without an open pull request, use the supplied target or fall back to origin/HEAD.

Use git for fetch and merge. Use gh only to read pull request data. The bundled scripts/update-context.sh selects the target and verifies the pull request context. A branch without a pull request stays local. This run never pushes a remote branch unless the operator explicitly requests that remote update with the helper's push command.