Why: Labeled secrets, excluded turns, and candidate replies need fields on the shared message contract, and context snapshots need a section breakdown plus a stable prefix hash.
Evidence: go test ./... in agent-system-core. New fields use omitempty, so existing golden JSON stays the same.
Impact: Clients that ignore unknown fields keep working. Assemblers can now mark a message excluded or as one candidate in a group.
Refs: docs/records/2026-10-04-sillytavern-review.md
weichen
bd632236c2