The _normalized_message_timestamp_for_key helper was preserving
microsecond precision (%.6f). When the same message is persisted by
both the WebUI sidecar JSON writer and the Hermes agent state.db
writer, their timestamps can differ by a few microseconds, causing
_session_message_merge_key to produce different keys for the same
logical message and letting both copies survive the dedup pass in
merge_session_messages_append_only.
Truncating to second-level granularity collapses sub-second drift to
the same key, so the duplicate is suppressed correctly.
Fixes#2616
manji
ff0aa69d5f