Fix site_id isolation for demo sites

- Auto-derive site_id from demo directory paths (demos/demo-site -> 'demo', demos/simple/test-simple -> 'simple')
- Add validation requiring explicit site_id for non-demo paths with helpful error messages
- Remove JavaScript 'demo' fallback and add proper error messaging for missing site_id
- Ensure each demo site uses isolated content namespace to prevent content mixing

Resolves issue where /sites/simple and /sites/demo both used site_id=demo

internal/content/enhancer.go

@@ -194,13 +194,89 @@ func (e *Enhancer) enhanceWithEngine(htmlContent []byte, filePath string) ([]byt
 
 // EnhanceInPlace performs in-place enhancement of static site files
 func (e *Enhancer) EnhanceInPlace(sitePath string, siteID string) error {
-	// Update the enhancer's site ID for this operation
+	// Use the provided siteID (derivation should happen at CLI level)
 	e.siteID = siteID
 
 	// Use EnhanceDirectory with same input and output (in-place)
 	return e.EnhanceDirectory(sitePath, sitePath)
 }
 
+// DeriveOrValidateSiteID automatically derives site_id for demo paths or validates for production
+func DeriveOrValidateSiteID(sitePath string, configSiteID string) string {
+	// Check if this is a demo path
+	if strings.Contains(sitePath, "/demos/") {
+		return deriveDemoSiteID(sitePath)
+	}
+
+	// For non-demo paths, return the configured site_id
+	// Validation of non-demo site_id will be handled at the CLI level
+	return configSiteID
+}
+
+// deriveDemoSiteID extracts site_id from demo directory structure
+func deriveDemoSiteID(sitePath string) string {
+	// Convert to absolute path and clean it
+	absPath, err := filepath.Abs(sitePath)
+	if err != nil {
+		absPath = sitePath
+	}
+	absPath = filepath.Clean(absPath)
+
+	// Look for patterns in demo paths:
+	// demos/demo-site_enhanced -> "demo"
+	// demos/simple/test-simple_enhanced -> "simple"
+	// demos/simple/dan-eden-portfolio -> "dan-eden"
+
+	parts := strings.Split(absPath, string(filepath.Separator))
+
+	// Find the demos directory index
+	demosIndex := -1
+	for i, part := range parts {
+		if part == "demos" {
+			demosIndex = i
+			break
+		}
+	}
+
+	if demosIndex == -1 || demosIndex >= len(parts)-1 {
+		// Fallback if demos not found in path
+		return "demo"
+	}
+
+	// Get the segment after demos/
+	nextSegment := parts[demosIndex+1]
+
+	// Handle different demo directory patterns:
+	if strings.HasPrefix(nextSegment, "demo-site") {
+		return "demo"
+	}
+
+	if nextSegment == "simple" && len(parts) > demosIndex+2 {
+		// For demos/simple/something, use the something part
+		finalSegment := parts[demosIndex+2]
+
+		// Extract meaningful name from directory
+		if strings.HasPrefix(finalSegment, "test-") {
+			// test-simple_enhanced -> simple
+			return "simple"
+		}
+		if strings.Contains(finalSegment, "dan-eden") {
+			return "dan-eden"
+		}
+
+		// Generic case: use the directory name as-is
+		return strings.TrimSuffix(finalSegment, "_enhanced")
+	}
+
+	// Default case: use the immediate subdirectory of demos/
+	result := strings.TrimSuffix(nextSegment, "_enhanced")
+
+	// Clean up common suffixes
+	result = strings.TrimSuffix(result, "-site")
+
+	return result
+}
+
 // copyFile copies a file from src to dst
 func (e *Enhancer) copyFile(src, dst string) error {
 	// Create directory for destination