Page-history snapshots are debounced/coalesced (one per 1–5 min window,
jobId=page.id). A human edit followed by an agent edit in the same window
collapsed into a single snapshot, losing both the pre-agent human state and
a deterministic record of the agent's result.
Two provenance-aware boundaries now bracket an agent intervention:
- Before: on a user->agent transition, onStoreDocument synchronously pins the
current (pre-agent) human content as its own history version tagged 'user',
inside the page-write transaction, before the agent overwrites it.
- After: agent stores enqueue an immediate (delay 0), source-keyed history job
(jobId=`${pageId}:agent`) so the agent's result snapshots deterministically
as 'agent' and a later human edit (jobId=page.id) cannot coalesce/retag it.
Also add an `id desc` tie-break to findPageLastHistory so "last history" stays
deterministic when two snapshots share a created_at, consistent with
findPageHistoryByPageId.
Known trade-offs (Variant 1): the delay-0 worker re-reads the row, leaving a
millisecond mis-tag window; multiple agent edits in one turn may yield multiple
versions. The reverse agent->human boundary is intentionally out of scope.
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