Add Claude plugin manifest (#24)

adding plugin json to be compatible with most recent claude marketplace
requirements

https://code.claude.com/docs/en/plugins-reference#plugin-manifest-schema

Author: jlow2397

plugins/braintrust/.claude-plugin/plugin.json

@@ -1,8 +1,21 @@
 {
+  "$schema": "https://json.schemastore.org/claude-code-plugin-manifest.json",
   "name": "braintrust",
+  "displayName": "Braintrust",
   "description": "Enables AI agents to use Braintrust for LLM evaluation, logging, and observability. Provides correct API usage, working examples, and helper scripts for common operations.",
   "version": "1.3.1",
   "author": {
-    "name": "Braintrust"
-  }
+    "name": "Braintrust",
+    "email": "support@braintrust.dev",
+    "url": "https://braintrust.dev"
+  },
+  "homepage": "https://braintrust.dev",
+  "repository": "https://github.com/braintrustdata/braintrust-claude-plugin",
+  "license": "MIT",
+  "keywords": [
+    "braintrust",
+    "evals",
+    "llm-evaluation",
+    "observability"
+  ]
 }