feat: add Claude Code plugin structure

Enables standalone skill installation via Claude Code marketplace.

- Add .claude-plugin/plugin.json with skill metadata
- Reference root-level SKILL.md via skills array

Author: CybotTM

.claude-plugin/plugin.json

@@ -0,0 +1,18 @@
+{
+  "name": "netresearch-branding",
+  "version": "1.2.1",
+  "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": ["."]
+}