feat: add MCP server configuration for Claude Code plugin integration

Add .mcp.json to configure the Unraid MCP server as a stdio-based MCP
server for Claude Code plugin integration. This allows Claude Code to
automatically start and connect to the server when the plugin is loaded.

- Type: stdio (standard input/output communication)
- Command: uv run unraid-mcp-server
- Forces stdio transport mode via UNRAID_MCP_TRANSPORT env var

.mcp.json

@@ -0,0 +1,12 @@
+{
+  "mcpServers": {
+    "unraid": {
+      "type": "stdio",
+      "command": "uv",
+      "args": ["run", "unraid-mcp-server"],
+      "env": {
+        "UNRAID_MCP_TRANSPORT": "stdio"
+      }
+    }
+  }
+}