This merge adds done-payload snapshot handling across the chat system (FN-3430), normalizing how final assistant messages are preserved and consumed in the dashboard hooks, plus it normalizes dashboard mailbox and user identity for inter-agent messaging (FN-3484) and introduces plugin workflow step
Fusion-Task-Id: FN-3430
Merges a batch of five commits delivering dashboard performance optimizations (new composite indexes on `tasks.column`/`tasks.updatedAt` and debounced detail fetches), hardened chat SSE parsing with new test coverage, restored typecheck gates, split-pane task-link navigation (FN-3191), and research
Fusion-Task-Id: FN-3243
