docs: fix mdl-entities CreatedDate example and improve MDL020 suggestion

mdl-entities.md used `CreatedDate: datetime` in two examples, which
triggers lint error MDL020 (system attribute name conflict). Changed
to `CreatedDate: autocreateddate` with an explanatory comment.

MDL020 suggestion now explains both options:
  - use the pseudo-type (autocreateddate) for the Mendix built-in field
  - choose a different name (e.g. EntryDate) for an unrelated date field

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

Author: ako

.claude/skills/mendix/mdl-entities.md

@@ -30,8 +30,10 @@ create persistent entity Module.Customer (
   IsActive: boolean default true,
 
   -- Date/Time
-  CreatedDate: datetime,
   BirthDate: date,
+  -- Use autocreateddate (not datetime) to record when the object was created.
+  -- 'CreatedDate' as a plain datetime triggers lint error MDL020.
+  CreatedDate: autocreateddate,
 
   -- Enumeration
   status: Module.CustomerStatus default Active,
@@ -255,7 +257,7 @@ create persistent entity Shop.Customer (
   Email: string(200) not null unique,
   Phone: string(20),
   IsActive: boolean default true,
-  CreatedDate: datetime
+  CreatedDate: autocreateddate
 );
 /
 

mdl/executor/cmd_enumerations.go

@@ -323,7 +323,7 @@ func ValidateEntity(stmt *ast.CreateEntityStmt) []linter.Violation {
 					DocumentType: "entity",
 					DocumentName: stmt.Name.String(),
 				},
-				Suggestion: fmt.Sprintf("Rename to avoid conflicts (e.g., 'Custom%s')", attr.Name),
+				Suggestion: fmt.Sprintf("To use the Mendix built-in audit field, declare it with the pseudo-type: '%s: Auto%s'. To store an unrelated date, choose a different name (e.g., 'EntryDate', 'RecordDate')", attr.Name, attr.Name),
 			})
 		}
 	}