Why: The Frontend groups scratch sessions under a separate sidebar category and
needs an explicit marker instead of inferring it from the project list.
Evidence: agent-exec run 2d662c46 (pi, success); go test ./internal/store ./internal/server pass.
Impact: listTasks, listConversations, conversationDetail and POST /api/scratch-sessions
return scratch: true for scratch conversations; other conversations keep the same JSON (omitempty).
Refs: Agent-System-Dev docs/superpowers/specs/2026-10-22-scratch-session-general-agent-design.md
Co-authored-by: Cursor <cursoragent@cursor.com>
