hook: agent-ledger.sh

Part 3 · EACH TOOL · Slide 49 of 60

Event: SubagentStop · 30 s.
Writes four things per finished lane: a row in COORD-AGENTS.md,
the lane's exact first message as briefs/agent-<id>.md
(commission transparency), a deduplicated receipt in
spend/ledger.md, and the lane's four-box card as records —
lane-authored lessons land PROPOSED, never live. Retires the
lane's gates section by its key. Writes are locked.

   lane ends ──► row + brief + receipt + card banked