Idle prefetch used the 50% growth floor and compacted agents with 35pt of headroom (coder-pluto-cc at 51.2%)

FIXED+PUSHED 2026-08-01 (sh.git d706e22). Idle prefetch shared FLOOR_PCT=50 with the growth tier, so it compacted agents with huge headroom purely for being idle. Measured: coder-pluto-cc compacted 10:58 (76.0%, escalation - correct), then AGAIN 11:24 on prefetch at 51.2%, 26min later - past the new 20min re-compact cooldown, so OPS-111 does not catch it. At 51.2% against an 86.8% auto-compact boundary there was 35pt of headroom; the compact reclaimed little and cost a full context reset. 'Compact now while it's free' was always false - a compact costs the agent its entire working context. Fix: PREFETCH_FLOOR_PCT=65.0, separate from FLOOR_PCT, chosen as the level from which one busy stretch can plausibly reach the busy-path nudge zone (nudges actually fired at 68-76% today). Below it, growth and the hard cap decide - those tiers have a reason beyond idleness. Also added a distinct skip reason so a below-prefetch-floor idle agent is not silently lumped in with 'already prefetched'. mem:project_monitor_context_venus_actuator.md