docs: add type sharing guidelines to PROJECT_GUIDELINES

Add guidelines for sharing types between handlers and templates to prevent
duplication and maintain consistency across the codebase.

PROJECT_GUIDELINES.md

@@ -155,6 +155,12 @@ VERIFY → Compile successfully
 - ✅ Place new files in **contextually appropriate directories**
 - ✅ Follow **KISS**, **DRY**, and **YAGNI** principles
 - ✅ Use **multiple, logical git commits** with clear messages
+- ❌ **NEVER duplicate types between handlers and templates** - define data types in handlers, reuse directly in templates
+- ✅ **DO share handler types with templates** - templates should use handlers.CollectionData, handlers.BookInfo, etc. directly
+- ❌ **NEVER create parallel type systems** - no templates.XxxData types for data that originates from handlers
+- ✅ **DO enhance handler types with template-specific fields** when needed (e.g., add Icon, FormattedDate fields to handlers structs)
+- ✅ **DO keep template-only types in templates package** - PageData, UnsafeHTML, and template-specific utilities are appropriate
+- ❌ **NEVER create conversion helper functions** to map between handler and template types - use handler types directly
 
 ### Configuration & Environment
 - ✅ If **.env is missing**, auto-generate secure values