fix(ci): stop skipping conflicts, and keep branches current

[catomean]

Ported from bitbaum/orangecat#645. The merge train optimised for merging
and had nothing to say about not merging — three different stuck states all
rendered as the same polite skip:

• A conflicted PR was treated exactly like "not ready yet" and passed over
  in silence, every sweep, forever. Nothing else will unstick it. Now logged to
  stderr and written to the job summary.
• Nothing kept branches current, so a PR could merge against a base it was
  never proven against, and conflicts surfaced hours after the merge that caused
  them. A BEHIND branch is now updated first — one per sweep, for the same
  reason only one PR merges per sweep — which surfaces conflicts minutes later
  instead of hours.
• "No checks reported yet" is transient for a PR opened seconds ago and
  PERMANENT for an old one: GitHub does not retroactively run workflows on a PR
  nobody has pushed to. Checkless PRs over 2h old are now surfaced instead of
  waiting patiently forever. (Found live: 10 such PRs in one repo.)

Behaviour verified against the real script with a fake gh on PATH — conflict
reported and not merged; behind-updated and not merged; clean still merges (no
regression); old checkless PR surfaced; brand-new one stays silent.

🤖 Generated with Claude Code