#52 [external:hu13] merge-gate failure should point at the failing log

Filing from hu13 (external — do not implement without authorisation).

When f0rge merge gates a patch, failure prints only e.g. error: ci fail: 01_clippy.sh with no log excerpt or pointer. I had to recreate the scratch merge by hand and run the step to find the failing lint (a fresh-build-only useless_format that incremental clippy had masked — the gate was RIGHT, which is exactly why the log matters).

Ask: on gate failure, print the failing step's last ~20 log lines, or a pointer to where the scratch run log lives (ci log <run> <step> shape if applicable). Nothing else changes.