feat: expose neural entity extraction APIs (v5.7.6 - Workshop request)

Addresses Workshop team's request for direct access to neural extraction classes.

**Changes:**

1. **New Exports** (src/index.ts):
   - `NeuralEntityExtractor` - Full extraction orchestrator
   - `SmartExtractor` - Entity type classifier (4-signal ensemble)
   - `SmartRelationshipExtractor` - Relationship type classifier
   - Types: `ExtractedEntity`, `ExtractionResult`, `RelationshipExtractionResult`, etc.

2. **Package.json Subpath Exports**:
   ```typescript
   // Enable direct imports:
   import { NeuralEntityExtractor } from '@soulcraft/brainy/neural/entityExtractor'
   import { SmartExtractor } from '@soulcraft/brainy/neural/SmartExtractor'
   import { SmartRelationshipExtractor } from '@soulcraft/brainy/neural/SmartRelationshipExtractor'
   ```

3. **New brain.extractEntities() Method** (brainy.ts:3254):
   - Alias for `brain.extract()` with clearer naming
   - Documented with examples and architecture details
   - 4-signal ensemble: ExactMatch (40%) + Embedding (35%) + Pattern (20%) + Context (5%)

4. **Comprehensive Documentation** (docs/neural-extraction.md):
   - Complete neural extraction guide (200+ lines)
   - API reference for all extraction classes
   - Performance optimization tips
   - Import preview mode documentation
   - Confidence scoring explanation
   - 42 NounType detection methods
   - Troubleshooting guide
   - Real-world examples

5. **README Updates**:
   - Added "Entity Extraction" section with examples
   - Links to neural extraction guide
   - Import preview mode link

**Features:**
- ⚡ Fast extraction: ~15-20ms per entity
- 🎯 4-signal ensemble architecture
- 📊 Format intelligence (Excel, CSV, PDF, YAML, DOCX, JSON, Markdown)
- 🌍 42 universal noun types + 127 verb types
- 💾 LRU caching built-in
- 🧪 Production-tested in import pipeline

**Usage:**

```typescript
// Simple API (recommended)
const entities = await brain.extractEntities('John Smith founded Acme Corp', {
  types: [NounType.Person, NounType.Organization],
  confidence: 0.7
})

// Advanced API (custom configuration)
import { SmartExtractor } from '@soulcraft/brainy'

const extractor = new SmartExtractor(brain, { minConfidence: 0.8 })
const result = await extractor.extract('CEO', {
  formatContext: { format: 'excel', columnHeader: 'Title' }
})
```

**Backward Compatible:** All existing APIs unchanged. New exports are pure additions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

README.md

@@ -135,6 +135,50 @@ const results = await brain.find({
 
 ---
 
+## Entity Extraction (NEW in v5.7.6)
+
+**Extract entities from text with AI-powered classification:**
+
+```javascript
+import { Brainy, NounType } from '@soulcraft/brainy'
+
+const brain = new Brainy()
+await brain.init()
+
+// Extract all entities
+const entities = await brain.extractEntities('John Smith founded Acme Corp in New York')
+// Returns:
+// [
+//   { text: 'John Smith', type: NounType.Person, confidence: 0.95 },
+//   { text: 'Acme Corp', type: NounType.Organization, confidence: 0.92 },
+//   { text: 'New York', type: NounType.Location, confidence: 0.88 }
+// ]
+
+// Extract with filters
+const people = await brain.extractEntities(resume, {
+  types: [NounType.Person],
+  confidence: 0.8
+})
+
+// Advanced: Direct access to extractors
+import { SmartExtractor } from '@soulcraft/brainy'
+
+const extractor = new SmartExtractor(brain, { minConfidence: 0.7 })
+const result = await extractor.extract('CEO', {
+  formatContext: { format: 'excel', columnHeader: 'Title' }
+})
+```
+
+**Features:**
+- 🎯 **4-Signal Ensemble** - ExactMatch (40%) + Embedding (35%) + Pattern (20%) + Context (5%)
+- 📊 **Format Intelligence** - Adapts to Excel, CSV, PDF, YAML, DOCX, JSON, Markdown
+- ⚡ **Fast** - ~15-20ms per extraction with LRU caching
+- 🌍 **42 Types** - Person, Organization, Location, Document, and 38 more
+
+**→ [Neural Extraction Guide](docs/neural-extraction.md)** | **[Import Preview Mode](docs/neural-extraction.md#import-preview-mode)**
+
+---
+
 ## From Prototype to Planet Scale
 
 **The same API. Zero rewrites. Any scale.**