Add Claude Code plugin support

- Add plugin.json manifest for Claude Code plugin installation
- Update README with Claude Code plugin installation instructions
- Update repository URLs to point to jiayuan7/andrej-karpathy-skills

This enables users to install the skill as a Claude Code plugin:
  claude plugins add https://github.com/jiayuan7/andrej-karpathy-skills

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugin.json

@@ -0,0 +1,12 @@
+{
+  "name": "andrej-karpathy-skills",
+  "displayName": "Karpathy Coding Guidelines",
+  "version": "1.0.0",
+  "description": "Behavioral guidelines to reduce common LLM coding mistakes, derived from Andrej Karpathy's observations on LLM coding pitfalls",
+  "author": "jiayuan7",
+  "license": "MIT",
+  "repository": "https://github.com/jiayuan7/andrej-karpathy-skills",
+  "skills": [
+    "skills/karpathy-guidelines/SKILL.md"
+  ]
+}