fix: use CLAUDE_PLUGIN_ROOT for portable MCP server configuration

Update .mcp.json to use  environment variable
for the --directory argument, ensuring the MCP server works correctly
regardless of where the plugin is installed.

This follows Claude Code plugin best practices for MCP server bundling.

.mcp.json

@@ -3,7 +3,7 @@
     "unraid": {
       "type": "stdio",
       "command": "uv",
-      "args": ["run", "unraid-mcp-server"],
+      "args": ["run", "--directory", "${CLAUDE_PLUGIN_ROOT}", "unraid-mcp-server"],
       "env": {
         "UNRAID_MCP_TRANSPORT": "stdio"
       }