feat(#22): implement brain query API

- Create brain module with service, controller, and DTOs
- POST /api/brain/query - Structured queries for tasks, events, projects
- GET /api/brain/context - Get current workspace context for agents
- GET /api/brain/search - Search across all entities
- Support filters: status, priority, date ranges, assignee, etc.
- 41 tests covering service (27) and controller (14)
- Integrated with AuthGuard, WorkspaceGuard, PermissionGuard

apps/api/src/brain/dto/index.ts

@@ -0,0 +1 @@
+export { BrainQueryDto, TaskFilter, EventFilter, ProjectFilter, BrainContextDto } from "./brain-query.dto";