26.8.0-trunk.17.0517.204725

runner-cleanup: drop redundant age-based _diag prune

Section 2b in the housekeeping pass (added in PR #920, merged as
f8c6fda3) already wipes ~/_diag/ on every runner each tick:

  for user in "${runner_users[@]}"; do
      ...
      note "wipe ${user}/_diag"
      run find "$diag" -mindepth 1 -delete
  done

The older section 4c was left behind:

  for user in "${runner_users[@]}"; do
      ...
      log "prune ${user}/_diag (older than ${diag_days}d)"
      run find "${home}/_diag" -type f \
          \( -name 'Runner_*.log' -o -name 'Worker_*.log' -o -name '*.log.gz' \) \
          -mtime "+${diag_days}" -delete
  done

By the time 4c runs, section 2b has already emptied ~/_diag/, so 4c
finds nothing to delete — it just emits a misleading "prune ..." log
line. Visible in the verbose output as 16 _diag lines per pass: 8
"wipe" + 8 "prune".

Drop the 4c block entirely and renumber the following docker
container-log section from 4d to 4c.