Repair Slack Scoped Historical Holes

Why

Slack targeted channel repair can fetch a scoped source archive that contains messages older than the saved per-channel cursor. Those historical holes must be emitted during the repair; otherwise the run can succeed while retained records still miss source archive keys.

What Changes

• Treat messages.resources scoped Slack runs as repair passes over the scoped archive.
• Ignore saved message cursors for the scoped archive while preserving normal cursor filtering for unscoped incremental runs.
• Add a regression test for a scoped archive row older than channel_last_ts.

Capabilities

• Modified: reference-implementation-architecture

Impact

• Touches the Slack reference connector and Slack runtime tests.
• Does not change PDPP Core, grant semantics, or non-Slack connectors.