Add mx create-project + mx check testing pattern to test-mdl skill

Document headless validation workflow: create a fresh blank project with
mx create-project, apply MDL changes, validate with mx check. Includes
CE0066 scenario test example and updated checklist.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ako

.claude/skills/test-mdl.md

@@ -35,7 +35,73 @@ Execute MDL commands against the test project:
 ./bin/mxcli -p mx-test-projects/test1-go-app/test1-go.mpr exec reference/mendix-repl/examples/doctype-tests/domain-model-examples.mdl
 ```
 
-### 4. Verify in Studio Pro
+### 4. Validate with mx check (Headless)
+
+Use `mx check` to validate projects without Studio Pro. This catches consistency errors (CE0066, CE1613, etc.) that would otherwise only show up when opening in Studio Pro.
+
+```bash
+# Find the mx binary
+MX=~/.mxcli/mxbuild/*/modeler/mx
+
+# Check the project
+$MX check /path/to/app.mpr
+```
+
+Expected output for a clean project:
+```
+Checking app for errors...
+The app contains: 0 errors.
+```
+
+### 5. Isolated Testing with mx create-project
+
+For testing changes in isolation without modifying existing test projects, create a fresh blank project:
+
+```bash
+# Create a fresh Mendix project in a temp directory
+cd /tmp/test-workspace
+$MX create-project
+
+# This creates a full Mendix project structure:
+#   App.mpr, mprcontents/, javascriptsource/, javasource/, etc.
+# IMPORTANT: Run from the target directory — it creates files in the CWD.
+
+# Apply MDL changes
+mxcli exec script.mdl -p /tmp/test-workspace/App.mpr
+
+# Validate
+$MX check /tmp/test-workspace/App.mpr
+```
+
+This pattern is ideal for:
+- **Regression testing**: verify a fix doesn't break `mx check`
+- **BSON validation**: confirm serialized structures are valid Mendix format
+- **Security testing**: verify GRANT/REVOKE produce correct access rules (CE0066)
+- **CI pipelines**: headless validation without Studio Pro
+
+#### Example: CE0066 scenario test
+
+```bash
+# Fresh project
+cd /tmp/ce0066-test && $MX create-project
+
+# Apply changes and validate
+cat > /tmp/test.mdl << 'EOF'
+CREATE MODULE ROLE MyFirstModule.Admin;
+CREATE OR MODIFY PERSISTENT ENTITY MyFirstModule.Order (
+    Code: String(50),
+    Total: Decimal
+);
+GRANT MyFirstModule.Admin ON MyFirstModule.Order (CREATE, DELETE, READ *, WRITE *);
+ALTER ENTITY MyFirstModule.Order ADD ATTRIBUTE Status String(50);
+EOF
+
+mxcli exec /tmp/test.mdl -p /tmp/ce0066-test/App.mpr
+$MX check /tmp/ce0066-test/App.mpr
+# Expected: 0 errors
+```
+
+### 6. Verify in Studio Pro
 
 After executing MDL:
 1. Open the MPR file in Mendix Studio Pro
@@ -132,6 +198,7 @@ go build -o bin/mxcli ./cmd/mxcli
 - [ ] MDL script executes without errors
 - [ ] `go build ./...` succeeds
 - [ ] `go test ./...` passes
-- [ ] Studio Pro opens project without errors
-- [ ] Created elements appear correctly in Studio Pro
+- [ ] `mx check` passes on a fresh project with the changes applied (0 errors)
+- [ ] Studio Pro opens project without errors (if available)
+- [ ] Created elements appear correctly in Studio Pro (if available)
 - [ ] BSON mapping documentation updated if new patterns discovered