## Why
Committing a Guardian review snapshot currently flushes and reloads the transcript from storage, even though the completed model context is already available in memory.
## What changed
- Build forkable checkpoints directly from live session state, preserving model history, retained Guardian context, world state, turn context, and token usage.
- Preserve compaction window metadata and MCP resource origins in the checkpoint.
- Recognize prior user turns inside compacted replacement history.
## Testing
Add round-trip checkpoint tests for legacy and thread-owned Guardian context without storage, including preservation after live history is mutated. Extend the review compaction test to assert that checkpoints do not load history from storage.
GitOrigin-RevId: cb2638ca93bba6ab4472de50c7e2913e1a2f25a8
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