Add README and setup documentation for Greptile plugin

- Add README.md with setup instructions for getting API key
- Document the GREPTILE_API_KEY environment variable requirement
- Add homepage, author URL, and keywords to plugin.json
- Update description to reflect Greptile as AI code review agent

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

external_plugins/greptile/.claude-plugin/plugin.json

@@ -1,7 +1,10 @@
 {
   "name": "greptile",
-  "description": "AI-powered codebase search and understanding. Query your repositories using natural language to find relevant code, understand dependencies, and get contextual answers about your codebase architecture.",
+  "description": "AI code review agent for GitHub and GitLab. View and resolve Greptile's PR review comments directly from Claude Code.",
   "author": {
-    "name": "Greptile"
-  }
+    "name": "Greptile",
+    "url": "https://greptile.com"
+  },
+  "homepage": "https://greptile.com/docs",
+  "keywords": ["code-review", "pull-requests", "github", "gitlab", "ai"]
 }