feat: add Claude Code plugin structure

Add .claude-plugin/plugin.json for native Claude Code plugin support.
This enables the skill to be installed directly without the marketplace bundle workaround.

Author: CybotTM

.claude-plugin/plugin.json

@@ -0,0 +1,19 @@
+{
+  "name": "typo3-core-contributions",
+  "version": "1.5.0",
+  "description": "TYPO3 Core contribution workflow (Forge, Gerrit, docs)",
+  "repository": "https://github.com/netresearch/typo3-core-contributions-skill",
+  "license": "MIT",
+  "author": {
+    "name": "Netresearch DTT GmbH",
+    "email": "info@netresearch.de"
+  },
+  "keywords": [
+    "typo3",
+    "core",
+    "contributions",
+    "gerrit",
+    "forge"
+  ],
+  "skills": ["."]
+}