Move kilocode_change check to package.json script following knip pattern

- Add check-kilocode-change script in packages/kilo-vscode/package.json
- Update CI to use the script instead of inlined grep
- Document the check in root AGENTS.md alongside knip

.github/workflows/test-vscode.yml

@@ -44,12 +44,5 @@ jobs:
         run: bun run knip
 
       - name: Check for kilocode_change markers
-        run: |
-          if grep -r --include='*' -l 'kilocode_change' packages/kilo-vscode/ packages/kilo-ui/; then
-            echo ""
-            echo "ERROR: Found 'kilocode_change' markers in kilo-specific packages."
-            echo "These packages are entirely Kilo Code additions and do not need kilocode_change markers."
-            echo "Please remove them from the files listed above."
-            exit 1
-          fi
-          echo "No kilocode_change markers found. All clear."
+        working-directory: packages/kilo-vscode
+        run: bun run check-kilocode-change