feat: Add Moodle External API Development skill (#6)

- Merged PR from LocNguyenSGU
- Updated skill count to 156
- Added skill entry to README and skills_index.json

skills_index.json

@@ -917,6 +917,12 @@
     "name": "writing-skills",
     "description": "Use when creating new skills, editing existing skills, or verifying skills work before deployment"
   },
+  {
+    "id": "moodle-external-api-development",
+    "path": "skills/moodle-external-api-development",
+    "name": "moodle-external-api-development",
+    "description": "Create custom external web service APIs for Moodle LMS. Use when implementing web services for course management, user tracking, quiz operations, or custom plugin functionality. Covers parameter validation, database operations, error handling, service registration, and Moodle coding standards."
+  },
   {
     "id": "xlsx-official",
     "path": "skills/xlsx-official",
@@ -929,4 +935,4 @@
     "name": "zapier-make-patterns",
     "description": "\"No-code automation democratizes workflow building. Zapier and Make (formerly Integromat) let non-developers automate business processes without writing code. But no-code doesn't mean no-complexity - these platforms have their own patterns, pitfalls, and breaking points.  This skill covers when to use which platform, how to build reliable automations, and when to graduate to code-based solutions. Key insight: Zapier optimizes for simplicity and integrations (7000+ apps), Make optimizes for power \""
   }
-]
+]