feat: add custom pluggable widget AIGC skill

Add a skill file that teaches Claude to autonomously create Mendix
pluggable widgets from natural language descriptions. Includes:
- Step-by-step scaffolding with correct widget id namespace format
- Complete property type reference (string, attribute, datasource, etc.)
- React component patterns and Mendix prop API usage
- Build, install, and troubleshooting guide

Key fix: widget id must include a lowercase widget name segment
(e.g. com.example.widgets.helloworld.HelloWorld) to match the JS
subdirectory the build tool creates, preventing 404 at runtime.

Author: engalar

.claude/skills/mendix/README.md

@@ -59,6 +59,7 @@ Page-specific patterns:
 | Skill | Purpose | Use When |
 |-------|---------|----------|
 | [generate-domain-model.md](generate-domain-model.md) | Complete domain model generation | Generating full domain models |
+| [create-custom-widget.md](create-custom-widget.md) | Custom pluggable widget AIGC | Creating custom React widgets from scratch |
 | [debug-bson.md](debug-bson.md) | BSON debugging | Troubleshooting SDK issues |
 
 ---
@@ -83,6 +84,8 @@ Load skills based on the task:
 | "Change widgets in bulk" | `bulk-widget-updates.md` |
 | "Reuse widgets across pages" | `fragments.md` |
 | "Define a fragment" | `fragments.md` |
+| "Create custom widget" | `create-custom-widget.md` |
+| "Build a pluggable widget" | `create-custom-widget.md` |
 
 ### For Error Recovery