refactor: restructure skill for Claude Code compatibility

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

Changes:
- Moved SKILL.md to skills/netresearch-branding/SKILL.md
- Moved references/, templates/, scripts/ directories
- Updated plugin.json with new structure and version 1.3.1

Author: CybotTM

.claude-plugin/plugin.json

@@ -1,18 +1,17 @@
 {
   "name": "netresearch-branding",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "description": "Apply Netresearch brand identity to web projects and documentation",
   "repository": "https://github.com/netresearch/netresearch-branding-skill",
   "license": "MIT",
   "author": {
     "name": "Netresearch DTT GmbH",
     "email": "info@netresearch.de"
   },
-  "keywords": [
-    "netresearch",
-    "branding",
-    "design",
-    "identity"
-  ],
-  "skills": ["."]
+  "skills": [
+    {
+      "name": "netresearch-branding",
+      "path": "skills/netresearch-branding"
+    }
+  ]
 }