fix(ci): stop skipping conflicts, and let a red base be repaired

[catomean]

Brings this repo in line with the rest of the fleet (bitbaum/orangecat#650
and the ports alongside it).

Two ways the merge train reported progress but could not report being stuck:

• 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 and
  written to the job summary; BEHIND branches are updated before merging.
• A red base trapped the PR that repairs it. "Never merge onto red" is right
  for an unrelated change and a deadlock when the PR is the fix — it froze
  maonakamoto/aoz-housing outright on 2026-08-07. The sweep now identifies
  which jobs are red and lets a PR through only if its own checks pass every
  one of them. Not a weakening: a PR's checks run on the merge result
  (refs/pull/N/merge), so green-on-those-jobs is direct evidence the post-merge
  base is better than the pre-merge base.

Still refused: a PR not covering the failing jobs, one covering only some, and a
base failure whose jobs cannot be identified.

Verified against the real script with a fake gh on PATH, including a base with
no CI history at all — where an else-only declaration would trip set -u and
kill the entire sweep.

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