fix(04): address tech debt from milestone audit

- Wrap intel content in <codebase-intel> tags in plan-phase.md
- Use ENTITY_DIR constant in isEntityFile() instead of hardcoded path

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

commands/gsd/plan-phase.md

@@ -293,7 +293,9 @@ Fill prompt with inlined content and spawn:
 {uat_content}
 
 **Codebase Intel (if exists):**
+<codebase-intel>
 {intel_content}
+</codebase-intel>
 
 </planning_context>
 

hooks/gsd-intel-index.js

@@ -551,7 +551,7 @@ function generateSummary(index, conventions) {
  * Check if a file path is an entity file
  */
 function isEntityFile(filePath) {
-  return filePath.includes('.planning/intel/entities/') &&
+  return filePath.includes(ENTITY_DIR + '/') &&
          filePath.endsWith('.md');
 }