weichen
7abfae1f6b
Why: parallel conversations in one project cannot see each other's transcripts, which already caused duplicate delivery of the same requirement (records/2026-09-30-lesson-parallel-session-duplication.md); artifacts/knowledge are already project-shared, so transcripts are the only missing channel. Evidence: permission formula and gate matrix designed fail-closed (project switch default off; per-conversation outbound/inbound flags default open/shared); tool design lands the "conversation history search" builtin planned in skeleton design 4.8. Impact: documents only; implementation follows in core/backend/frontend under Dev task feature/cross-session-reading. Refs: proposals/2026-09-29-system-skeleton-design-and-plan.md 4.8/4.9; records/2026-09-30-lesson-parallel-session-duplication.md.