[source-cin7] Add incremental sync for the `sales_list` stream

> 🤖 *This issue was generated by an AI Agent.*

## What's missing

`sales_list` is full-refresh only despite the API exposing an `Updated` timestamp usable as a cursor.

## Verified on `master`

`airbyte-integrations/connectors/source-cin7/manifest.yaml` contains zero occurrences of `incremental_sync`.

## Origin

Closed as stale: #67027 (@ryanmcg2203), reviewed 2025-10-09 with no author response. That PR proposed a `DatetimeBasedCursor` on `cursor_field: Updated`.

Filed to keep a record of the gap. The PR was closed for inactivity, not on merit — credit to the original contributor if this is picked up.

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/13178

Start with airbyte-integrations/connectors/source-cin7/manifest.yaml and inspect the source-cin7 stream configuration, which currently has no incremental_sync entries. Review the DatetimeBasedCursor proposal in closed issue #67027, especially its use of Updated as cursor_field. Done means sales_list no longer performs only full refreshes and uses Updated for incremental synchronization.

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