docs: add TypeScript domain extraction guideline to PROJECT_GUIDELINES

- Add principle for extracting domain concepts/types only when clearly beneficial
- Emphasize YAGNI approach to avoid over-engineering TypeScript code
- Allow sensible extraction when it reduces duplication or complexity

PROJECT_GUIDELINES.md

@@ -139,6 +139,7 @@ VERIFY → Compile successfully
 - ✅ **Use procedural/imperative style** as your default
 - ✅ **Borrow functional techniques** when they simplify code
 - ✅ **Avoid ideological purity** - the best paradigm is the one that fits the problem
+- ✅ **Extract domain concepts/types only when clearly beneficial** - apply YAGNI, avoid over-engineering
 - ✅ **Render initial data server-side** in Go templates for fast page loads
 - ✅ **Use JavaScript/HTMX for CRUD operations** (create, update, delete)
 - ✅ **Ensure progressive enhancement** - pages work without JavaScript