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/brain.controller.ts

@@ -0,0 +1,48 @@
+import {
+  Controller,
+  Get,
+  Post,
+  Body,
+  Query,
+  UseGuards,
+} from "@nestjs/common";
+import { BrainService } from "./brain.service";
+import { BrainQueryDto, BrainContextDto } from "./dto";
+import { AuthGuard } from "../auth/guards/auth.guard";
+import { WorkspaceGuard, PermissionGuard } from "../common/guards";
+import { Workspace, Permission, RequirePermission } from "../common/decorators";
+
+@Controller("brain")
+@UseGuards(AuthGuard, WorkspaceGuard, PermissionGuard)
+export class BrainController {
+  constructor(private readonly brainService: BrainService) {}
+
+  @Post("query")
+  @RequirePermission(Permission.WORKSPACE_ANY)
+  async query(
+    @Body() queryDto: BrainQueryDto,
+    @Workspace() workspaceId: string
+  ) {
+    return this.brainService.query({ ...queryDto, workspaceId });
+  }
+
+  @Get("context")
+  @RequirePermission(Permission.WORKSPACE_ANY)
+  async getContext(
+    @Query() contextDto: BrainContextDto,
+    @Workspace() workspaceId: string
+  ) {
+    return this.brainService.getContext({ ...contextDto, workspaceId });
+  }
+
+  @Get("search")
+  @RequirePermission(Permission.WORKSPACE_ANY)
+  async search(
+    @Query("q") searchTerm: string,
+    @Query("limit") limit: string,
+    @Workspace() workspaceId: string
+  ) {
+    const parsedLimit = limit ? Math.min(parseInt(limit, 10) || 20, 100) : 20;
+    return this.brainService.search(workspaceId, searchTerm || "", parsedLimit);
+  }
+}