Enable Renovate nix support with flake.lock maintenance

This lets Renovate periodically refresh flake.lock, keeping the
nixpkgs and agru inputs of the development shell up to date.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/renovate.json

@@ -36,5 +36,11 @@
 	],
 	"pre-commit": {
 		"enabled": true
+	},
+	"nix": {
+		"enabled": true,
+		"lockFileMaintenance": {
+			"enabled": true
+		}
 	}
 }