## What changed
Replace duplicated content-kind strings with constants shared by fragment producers and consumers, including authorization checks, forked-history filtering, and memory instructions. Share the compaction summary kind between core and Guardian's transcript filter.
## Testing
Add a regression test verifying that Guardian excludes compaction summaries while retaining surrounding user messages, including a user message with identical text.
GitOrigin-RevId: 36dbdf0f82c61b52d2571ac1c392fb151c98151b