docs(23-brainstormer-flow): create phase plan — 4 plans across 3 waves
Plan 00 (Wave 0): DB migration for message_type, shared types/validators, test stubs
Plan 01 (Wave 1): Server — addSystemMessage, handoff route, status-update route
Plan 02 (Wave 1): UI — ChatSpecCard, ChatHandoffIndicator, ChatTaskCreatedBadge, ChatStatusUpdateBadge, useBrainstormerDefault
Plan 03 (Wave 2): Wiring — ChatMessage dispatch, ChatMessageList propagation, ChatPanel brainstormer default, chatApi handoff
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>