/**
 * Append a throwable's message to an event Body so the CAUSE ships with the fact.
 * The boot mount/launch handlers previously evented a fixed string and buried the
 * real error in exception.message, where agent-facing surfaces and Body-matching
 * log queries never see it — making a benign boot race indistinguishable from a
 * fatal "entrypoint not found". The base message stays untouched (existing
 * dashboards match on it); the cause rides after a stable " | cause: " separator.
 */
export declare function appendCause(message: string, err: unknown, limit?: number): string;
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