Put the restored PostgreSQL socket on a short path

Why: The restored postmaster refused to start because the results-directory socket path exceeded the 107-byte Unix socket limit.
Evidence: restore/postgres.log from backup-20261012-150713 says the socket path is too long (SELF). bash -n passed.
Impact: The restored instance listens on /tmp/u7bk-18997, and cleanup removes that directory.
Refs: feature/u7-ops-drills
Co-authored-by: Cursor <cursoragent@cursor.com>

tests/ops/run_backup_drill.sh

@@ -57,6 +57,7 @@ cleanup() {
     "$PG_CTL" -D "$RESTORE_PG" stop -m fast >/dev/null 2>&1 || true
     rm -rf "$RESTORE_PG"
   fi
+  rm -rf /tmp/u7bk-18997
   if [[ -n "$ASIDE" && -d "$ASIDE" ]]; then rm -rf "$ASIDE"; fi
   if [[ "$OWNED" -eq 1 ]]; then ops_pg_remove "$ROOT" "$NAME" || true; fi
   exit "$ec"
@@ -210,7 +211,8 @@ ops_kill_checked "$APP_PIDFILE" "127.0.0.1:$APP_PORT" TERM || true
 
 echo "[backup] restore to new data directory on $RESTORE_PG_PORT"
 RESTORE_PG="$RESULTS/restore/pgdata"
-mkdir -p "$RESTORE_PG" "$RESULTS/restore/sock"
+mkdir -p "$RESTORE_PG" /tmp/u7bk-18997
+chmod 700 /tmp/u7bk-18997
 cp -a "$BASE/." "$RESTORE_PG/"
 cat >>"$RESTORE_PG/postgresql.auto.conf" <<EOF
 archive_mode = 'off'
@@ -220,7 +222,7 @@ rm -f "$RESTORE_PG/postmaster.pid" "$RESTORE_PG/standby.signal"
 touch "$RESTORE_PG/recovery.signal"
 chmod 700 "$RESTORE_PG"
 "$PG_CTL" -D "$RESTORE_PG" -l "$RESULTS/restore/postgres.log" -t 600 -w \
-  -o "-p ${RESTORE_PG_PORT} -k ${RESULTS}/restore/sock" start
+  -o "-p ${RESTORE_PG_PORT} -k /tmp/u7bk-18997" start
 export PGPORT="$RESTORE_PG_PORT"
 recovered=0
 for _ in $(seq 1 300); do