refactor: restructure skill for Claude Code compatibility

Move skill files to skills/php-modernization/ subdirectory structure.
This fixes installation in Claude Code which requires skills in subdirectories.

Changes:
- Moved SKILL.md to skills/php-modernization/SKILL.md
- Moved references/, templates/, scripts/ directories
- Updated plugin.json with new structure and version 1.4.1

Author: CybotTM

.claude-plugin/plugin.json

@@ -1,19 +1,17 @@
 {
   "name": "php-modernization",
-  "version": "1.4.0",
+  "version": "1.4.1",
   "description": "PHP 8.x modernization patterns with type safety and PHPStan",
   "repository": "https://github.com/netresearch/php-modernization-skill",
   "license": "MIT",
   "author": {
     "name": "Netresearch DTT GmbH",
     "email": "info@netresearch.de"
   },
-  "keywords": [
-    "php",
-    "modernization",
-    "php8",
-    "phpstan",
-    "type-safety"
-  ],
-  "skills": ["."]
+  "skills": [
+    {
+      "name": "php-modernization",
+      "path": "skills/php-modernization"
+    }
+  ]
 }