Why: Accounts without project.create need an isolated general-agent chat; each
scratch session is a private shell project (settings host.kind=scratch) with
exactly one cross-private conversation.
Evidence: agent-exec run 9b3d31a3 (pi, success); go test ./internal/store ./internal/server -run Scratch pass.
Impact: New POST /api/scratch-sessions (no project.create needed); project lists
omit scratch; second conversation under scratch 409; client writes cannot add or
remove the scratch marker; POST /api/projects rejects the marker.
Refs: Agent-System-Dev docs/superpowers/specs/2026-10-22-scratch-session-general-agent-design.md
Co-authored-by: Cursor <cursoragent@cursor.com>
