A pool rents at whatever the market offered when each pod was started, so one
fleet can hold a $0.35 card and a $0.79 card doing identical work. Shedding the
cheap one saves less than half as much, and which to prefer is the operator's
call, not ours — hence a checkbox rather than a new default.

With scale_down_costliest_first the dearest surplus pod goes first, tie-broken
by the idlest and then the coldest cache. Cost decides WHICH pod goes, never
whether one goes, and a dear pod with requests on it is drained rather than
dropped — so choosing it costs time, not work. Worth weighing before switching
on: an on-demand pod costs more than a spot pod, so "dearest first" prefers to
keep the interruptible ones.

Also fixes the default tie-break, which was inverted when scale-down was
written: `-last_used` picked the pod used MOST recently, discarding the warmest
prefix cache of the set. It now retires the coldest.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PxYHWbCAjjjD7A1rhFKtaW