Function verify_event_chain 

pub fn verify_event_chain(events: &[AgentEvent]) -> ChainVerificationResult

Verify the integrity of an ordered list of agent events. Verifies that content_hash fields are non-empty and that prev_hash chain linkage between consecutive events is valid. Note: event content_hash is computed from the operation’s source data (memory content or query string), not from the event payload JSON, so we verify it is present but do not recompute it.