Adds an optional occurred_at (ISO-8601 date/datetime) parameter to the
hindsight_retain tool schema, threaded into the retain item's timestamp
field. When absent, the item timestamp defaults to the configured event
clock (base from PR #82928 by @ragingbulld, authorship preserved) so the
Hindsight server can resolve relative time phrases; previously no item
timestamp was ever sent and temporal memories landed with null
occurred_start/occurred_end.
Fixes#93568. Salvages #82928.
Teknium
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