fix: correct license to Apache-2.0 in plugin.json

The project uses Apache License 2.0, not AGPL-3.0. Update plugin.json to reflect the correct license.

Author: amondnet

.claude-plugin/plugin.json

@@ -7,7 +7,7 @@
   },
   "homepage": "https://github.com/grafana/mcp-grafana",
   "repository": "https://github.com/grafana/mcp-grafana",
-  "license": "AGPL-3.0",
+  "license": "Apache-2.0",
   "mcpServers": {
     "grafana": {
       "command": "${CLAUDE_PLUGIN_ROOT}/.claude-plugin/install-binary.sh",