hook: agent-ledger.sh

Part 3 · EACH TOOL · Slide 49 of 60

Event: SubagentStop · 30 s.
Writes three things per finished lane: a row in
COORD-AGENTS.md, the lane's exact first message as
briefs/agent-<id>.md (commission transparency), and a
deduplicated receipt in spend/ledger.md. Writes are locked.

   lane ends ──► ledger row + brief + spend receipt