Implement unified Discoverer + Enhancer architecture

- Rename AutoEnhancer to Discoverer with clear element discovery focus
- Implement unified enhancement pipeline in Enhancer:
  * Phase 1: Element Discovery (configurable, respects existing insertr classes)
  * Phase 2: ID Generation via engine
  * Phase 3: Content Injection via engine
- Add EnhancementConfig and DiscoveryConfig for flexible configuration
- Update all method names and references (discoverNode, DiscoveryResult, etc.)
- Support both manual class insertion and automatic discovery
- Maintain single enhance command interface while providing unified internal pipeline
- Update all constructors to use new configuration-based approach

This establishes the clean Discoverer + Enhancer architecture discussed, with discovery as configurable first phase and enhancement as unified pipeline.

internal/content/site_manager.go

@@ -37,7 +37,7 @@ func NewSiteManager(contentClient engine.ContentClient, backupDir string, devMod
 
 	return &SiteManager{
 		sites:     make(map[string]*SiteConfig),
-		enhancer:  NewEnhancer(contentClient, ""), // siteID will be set per operation
+		enhancer:  NewDefaultEnhancer(contentClient, ""), // siteID will be set per operation
 		backupDir: backupDir,
 		devMode:   devMode,
 	}