Fix Slack Channel Coverage Highwater

Slack retained historical records for an engineering channel while current connector runs no longer enumerated that channel from the slackdump archive. The run still completed successfully with no known gaps, so downstream clients could mistake stale partial coverage for current absence.

The observed failure is a coverage-honesty bug, not an MCP client bug and not grant filtering. The owner-token view can see historical Slack records for a channel whose current slackdump archive no longer includes the channel inventory or messages. Recent runs succeeded with no known gaps.

When a connector collects an append-only stream from source partitions that can appear independently, the connector SHALL NOT report a clean successful run when a previously observed partition is absent from the current source inventory. The connector SHALL emit an explicit bounded diagnostic identifying the missing partition and affected stream.