audit-logger: record the event's own x-event-id (ADR 0047)

The trail's id is now the event's x-event-id — the handle a reader dedups
the at-least-once stream on — not the type@time placeholder the first cut
used. Carries causation/schema through when present.

Claude-Session: https://claude.ai/code/session_01LrgweAeERJYBg88c5cKDzF

modules/audit-logger/audit.ts

@@ -11,15 +11,18 @@ export function auditLogPath(env: NodeJS.ProcessEnv = process.env): string {
   return env.AUDIT_LOG ?? "/var/lib/audit-logger/audit.log";
 }
 
-/** Append an event to the trail as one JSON line, keeping the metadata an audit needs first. */
+/** Append an event to the trail as one JSON line, keeping the metadata an audit needs first. The id
+ *  is the event's own x-event-id (ADR 0047) — the handle a reader dedups the at-least-once trail on. */
 export async function record(event: Event, path: string): Promise<void> {
   const line =
     JSON.stringify({
-      id: event.type + "@" + event.at, // a stable-ish key until x-event-id headers land (ADR 0047)
+      id: event.id,
       type: event.type,
       source: event.source,
       node: event.node,
       at: event.at,
+      ...(event.causationId ? { causationId: event.causationId } : {}),
+      ...(event.schema ? { schema: event.schema } : {}),
       body: event.body,
     }) + "\n";
   await mkdir(dirname(path), { recursive: true }).catch(() => {});