[FEATURE]: Add a transcript revision token for incremental consumers

[FEATURE]: Add a transcript revision token for incremental consumers

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Describe the enhancement you want to request

Incremental consumers of session transcripts (e.g. external integrations that sync or archive conversations) currently cannot detect when a transcript they already read has changed. I hit this while developing an OpenCode session-sync plugin (related work in nowledge-co/community#522).

What exists today: the legacy and v2 session message endpoints both expose pagination cursors. Those cursors identify a message position and ordering, but there is no value that identifies the revision of the whole transcript. A client can keep paging from a cursor, but it cannot detect that OpenCode later changed or removed an earlier message or part. Compaction may also prune old tool-result content after a client has already read it.

Ask: expose a monotonic transcript revision (e.g. an ETag, revision number, or checkpoint token) with message-list responses, or via a dedicated endpoint. Incremental consumers could compare the revision and rescan only when previously read content changes.

I checked the legacy and v2 session message endpoints and could not find an existing revision/ETag/checkpoint API. I also found #19567 (durable event export / replay surface), which is related but distinct: that request is about an append-only event stream, while this one is a lightweight revision signal on the existing message-list surface.

Start by inspecting the legacy and v2 session message endpoints and how their pagination cursors are produced, then review how transcript changes and compaction affect returned messages. Define and document a monotonic revision, ETag, or checkpoint surface that incremental consumers can compare and use to trigger rescans when previously read content changes.