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>

pkg/config/config.go

@@ -24,6 +24,7 @@ type Config struct {
 	Slurp     SlurpConfig     `yaml:"slurp"`
 	Security  SecurityConfig  `yaml:"security"`
 	WHOOSHAPI WHOOSHAPIConfig `yaml:"whoosh_api"`
+	LightRAG  LightRAGConfig  `yaml:"lightrag"`
 }
 
 // AgentConfig defines agent-specific settings
@@ -161,6 +162,15 @@ type WHOOSHAPIConfig struct {
 	Enabled bool   `yaml:"enabled"`
 }
 
+// LightRAGConfig defines LightRAG RAG service settings
+type LightRAGConfig struct {
+	Enabled     bool          `yaml:"enabled"`
+	BaseURL     string        `yaml:"base_url"`
+	Timeout     time.Duration `yaml:"timeout"`
+	APIKey      string        `yaml:"api_key"`
+	DefaultMode string        `yaml:"default_mode"` // naive, local, global, hybrid
+}
+
 // LoadFromEnvironment loads configuration from environment variables
 func LoadFromEnvironment() (*Config, error) {
 	cfg := &Config{
@@ -270,6 +280,13 @@ func LoadFromEnvironment() (*Config, error) {
 			Token:   os.Getenv("WHOOSH_API_TOKEN"),
 			Enabled: getEnvBoolOrDefault("WHOOSH_API_ENABLED", false),
 		},
+		LightRAG: LightRAGConfig{
+			Enabled:     getEnvBoolOrDefault("CHORUS_LIGHTRAG_ENABLED", false),
+			BaseURL:     getEnvOrDefault("CHORUS_LIGHTRAG_BASE_URL", "http://127.0.0.1:9621"),
+			Timeout:     getEnvDurationOrDefault("CHORUS_LIGHTRAG_TIMEOUT", 30*time.Second),
+			APIKey:      os.Getenv("CHORUS_LIGHTRAG_API_KEY"),
+			DefaultMode: getEnvOrDefault("CHORUS_LIGHTRAG_DEFAULT_MODE", "hybrid"),
+		},
 	}
 
 	// Validate required configuration