standing-pr update: bail while a release is untagged, and reconcile from the latest main #799

Part of #790 (option 1). It's a code change, so it doesn't wait on the workflow work. Start after #794 merges, since both change runStandingPRUpdate.

Problem

An update that runs while a standing-PR publish is in flight sees the merged release commit's bumped package.json before the release's tags exist, and double-bumps. The mid-run guard in runStandingPRUpdate only bows out when the release commit is HEAD, so a feeder commit on top defeats it.

Fix

• Bail while a release is untagged. The update bows out, with a reason, while main holds a release commit whose release tags don't exist yet. The signal is a merged standing PR's manifest tags missing from the remote, or a direct release commit with none of its tags.
• But don't stall updates after a failure. It proceeds anyway when that release has a failure report (the publish failed, so the next release supersedes it). It also proceeds when the release commit is older than a backstop, about two hours, so a publish that crashed without reporting can't stall updates for good. Log a warning when the backstop applies.
• Reconcile against the latest main. Make the reconcile run by refresh-after-release analyse origin/<base> rather than the job's checkout. Today the dry run happens before resetReleaseBranch fetches, so in a publish job it analyses the tagged merge commit and can close a standing PR an update just opened.
• Restore the publish jobs' reconcile. Then end the example's publish and retry-publish jobs with refresh-after-release again, so the standing PR is rebuilt once the tags exist.

Acceptance criteria

• Through runStandingPRUpdate, with the fake forge and fake git:
  • an untagged release commit with a feeder commit on top returns a noop with the reason;
  • the same with a failure report proceeds;
  • the same past the backstop proceeds with a warning.
• A reconcile run inside a publish job's checkout analyses origin/main, and rebuilds rather than closes a standing PR that holds a newer feeder.