Add marketplace.json and fix plugin structure

- Add marketplace.json required for /plugin marketplace add
- Fix skill path to use ./ prefix (relative to plugin root)
- Update README with correct two-step install process:
  1. /plugin marketplace add forrestchang/andrej-karpathy-skills
  2. /plugin install andrej-karpathy-skills@karpathy-skills

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

.claude-plugin/marketplace.json

@@ -0,0 +1,29 @@
+{
+  "name": "karpathy-skills",
+  "id": "karpathy-skills",
+  "owner": {
+    "name": "forrestchang"
+  },
+  "metadata": {
+    "description": "Behavioral guidelines to reduce common LLM coding mistakes, derived from Andrej Karpathy's observations",
+    "version": "1.0.0"
+  },
+  "plugins": [
+    {
+      "name": "andrej-karpathy-skills",
+      "source": "./",
+      "description": "Behavioral guidelines to reduce common LLM coding mistakes: Think Before Coding, Simplicity First, Surgical Changes, Goal-Driven Execution",
+      "version": "1.0.0",
+      "author": {
+        "name": "forrestchang"
+      },
+      "keywords": [
+        "guidelines",
+        "best-practices",
+        "coding",
+        "karpathy"
+      ],
+      "category": "workflow"
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -7,6 +7,6 @@
   "license": "MIT",
   "repository": "https://github.com/forrestchang/andrej-karpathy-skills",
   "skills": [
-    "../skills/karpathy-guidelines/SKILL.md"
+    "./skills/karpathy-guidelines/SKILL.md"
   ]
 }