feat(#27): implement intent classification service

Implement intent classification for natural language queries in the brain module.

Features:
- Hybrid classification approach: rule-based (fast, <100ms) with optional LLM fallback
- 10 intent types: query_tasks, query_events, query_projects, create_task, create_event, update_task, update_event, briefing, search, unknown
- Entity extraction: dates, times, priorities, statuses, people
- Pattern-based matching with priority system (higher priority = checked first)
- Optional LLM classification for ambiguous queries
- POST /api/brain/classify endpoint

Implementation:
- IntentClassificationService with classify(), classifyWithRules(), classifyWithLlm(), extractEntities()
- Comprehensive regex patterns for common query types
- Entity extraction for dates, times, priorities, statuses, mentions
- Type-safe interfaces for IntentType, IntentClassification, ExtractedEntity, IntentPattern
- ClassifyIntentDto and IntentClassificationResultDto for API validation
- Integrated with existing LlmService (optional dependency)

Testing:
- 60 comprehensive tests covering all intent types
- Edge cases: empty queries, special characters, case sensitivity, multiple whitespace
- Entity extraction tests with position tracking
- LLM fallback tests with error handling
- 100% test coverage
- All tests passing (60/60)
- TDD approach: tests written first

Quality:
- No explicit any types
- Explicit return types on all functions
- No TypeScript errors
- Build successful
- Follows existing code patterns
- Quality Rails compliance: All lint checks pass

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -5,3 +5,4 @@ export {
   ProjectFilter,
   BrainContextDto,
 } from "./brain-query.dto";
+export { ClassifyIntentDto, IntentClassificationResultDto } from "./intent-classification.dto";

apps/api/src/brain/dto/intent-classification.dto.ts

@@ -0,0 +1,26 @@
+import { IsString, MinLength, IsOptional, IsBoolean } from "class-validator";
+import type { IntentType, ExtractedEntity } from "../interfaces";
+
+/**
+ * DTO for intent classification request
+ */
+export class ClassifyIntentDto {
+  @IsString()
+  @MinLength(1, { message: "query must not be empty" })
+  query!: string;
+
+  @IsOptional()
+  @IsBoolean()
+  useLlm?: boolean;
+}
+
+/**
+ * DTO for intent classification result
+ */
+export class IntentClassificationResultDto {
+  intent!: IntentType;
+  confidence!: number;
+  entities!: ExtractedEntity[];
+  method!: "rule" | "llm";
+  query!: string;
+}