open abstract method
Opens a PR for branch (already pushed with -u) against baseBranch,
run from workDir. Returns the PullRequestRef on success, or a
PrOpenFailure (never throws — the land step records either on the bead).
Implementation
Future<PullRequestResult> open({
required String workDir,
required String branch,
required String baseBranch,
required String title,
String body,
});