Add LightRAG MCP integration for RAG-enhanced AI reasoning

This commit integrates LightRAG (Retrieval-Augmented Generation) MCP server
support into CHORUS, enabling graph-based knowledge retrieval to enrich AI
reasoning and context resolution.

## New Components

1. **LightRAG Client** (pkg/mcp/lightrag_client.go)
   - HTTP client for LightRAG MCP server
   - Supports 4 query modes: naive, local, global, hybrid
   - Health checking, document insertion, context retrieval
   - 277 lines with comprehensive error handling

2. **Integration Tests** (pkg/mcp/lightrag_client_test.go)
   - Unit and integration tests
   - Tests all query modes and operations
   - 239 lines with detailed test cases

3. **SLURP Context Enricher** (pkg/slurp/context/lightrag.go)
   - Enriches SLURP context nodes with RAG data
   - Batch processing support
   - Knowledge base building over time
   - 203 lines

4. **Documentation** (docs/LIGHTRAG_INTEGRATION.md)
   - Complete integration guide
   - Configuration examples
   - Usage patterns and troubleshooting
   - 350+ lines

## Modified Components

1. **Configuration** (pkg/config/config.go)
   - Added LightRAGConfig struct
   - Environment variable support (5 variables)
   - Default configuration with hybrid mode

2. **Reasoning Engine** (reasoning/reasoning.go)
   - GenerateResponseWithRAG() - RAG-enriched generation
   - GenerateResponseSmartWithRAG() - Smart model + RAG
   - SetLightRAGClient() - Client configuration
   - Non-fatal error handling (graceful degradation)

3. **Runtime Initialization** (internal/runtime/shared.go)
   - Automatic LightRAG client setup
   - Health check on startup
   - Integration with reasoning engine

## Configuration

Environment variables:
- CHORUS_LIGHTRAG_ENABLED (default: false)
- CHORUS_LIGHTRAG_BASE_URL (default: http://127.0.0.1:9621)
- CHORUS_LIGHTRAG_TIMEOUT (default: 30s)
- CHORUS_LIGHTRAG_API_KEY (optional)
- CHORUS_LIGHTRAG_DEFAULT_MODE (default: hybrid)

## Features

- ✅ Optional and non-blocking (graceful degradation)
- ✅ Four query modes for different use cases
- ✅ Context enrichment for SLURP system
- ✅ Knowledge base building over time
- ✅ Health monitoring and error handling
- ✅ Comprehensive tests and documentation

## Testing

LightRAG server tested at http://127.0.0.1:9621
- Health check: ✅ Passed
- Query operations: ✅ Tested
- Integration points: ✅ Verified

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

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

internal/runtime/shared.go

@@ -21,6 +21,7 @@ import (
 	"chorus/pkg/dht"
 	"chorus/pkg/election"
 	"chorus/pkg/health"
+	"chorus/pkg/mcp"
 	"chorus/pkg/metrics"
 	"chorus/pkg/prompt"
 	"chorus/pkg/shhh"
@@ -682,5 +683,26 @@ func initializeAIProvider(cfg *config.Config, logger *SimpleLogger) error {
 		reasoning.SetDefaultSystemPrompt(d)
 	}
 
+	// Initialize LightRAG client if enabled
+	if cfg.LightRAG.Enabled {
+		lightragConfig := mcp.LightRAGConfig{
+			BaseURL: cfg.LightRAG.BaseURL,
+			Timeout: cfg.LightRAG.Timeout,
+			APIKey:  cfg.LightRAG.APIKey,
+		}
+		lightragClient := mcp.NewLightRAGClient(lightragConfig)
+
+		// Test connectivity
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		if lightragClient.IsHealthy(ctx) {
+			reasoning.SetLightRAGClient(lightragClient)
+			logger.Info("📚 LightRAG RAG system enabled - Endpoint: %s, Mode: %s",
+				cfg.LightRAG.BaseURL, cfg.LightRAG.DefaultMode)
+		} else {
+			logger.Warn("⚠️ LightRAG enabled but server not healthy at %s", cfg.LightRAG.BaseURL)
+		}
+	}
+
 	return nil
 }