learn: subagent_pressure never fires (spawn tool is now Agent, denominator counts every transcript)

source_claude.go counts "name":"task", but spawns are now tool_use blocks named Agent: 135 to 379 real spawns a week in my transcripts, 0 detected (checked at ed37ab1). SessionsScanned counts every transcript opened, not sessions with turns in the window (5,178 vs about 240 for one 7-day run), so pressure rounds to zero even with detection fixed. Before repairing the count: is a spawn waste? Delegating to a cheaper model saves cost, and over the same month spawns per session fell while spawns per turn rose. A measure that separates the two cases: the share of subagent tokens on the parent's tier or a more expensive one, plus the receipt size that re-enters the parent.