- `subagent_ended` carries `targetSessionKey` (identity — this matches `subagent_spawned.childSessionKey`), `targetKind` (`"subagent"` or `"acp"`), `reason`, optional `outcome` (`"ok"`, `"error"`, `"timeout"`, `"killed"`, `"reset"`, or `"deleted"`), optional `error`, `runId`, `endedAt`, `accountId`, and `sendFarewell`. It does **not** include `agentId` or `childSessionKey`; use `targetSessionKey` to correlate with the corresponding `subagent_spawned` event.