fix: set valid package-ecosystem values in dependabot.yml (#1)

The stock template was committed with package-ecosystem left empty,
which Dependabot rejects when parsing the config. Track the two
ecosystems present in the repo: GitHub Actions workflows and the
npm packages in site/.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: funsaized

.github/dependabot.yml

@@ -1,11 +1,14 @@
-# To get started with Dependabot version updates, you'll need to specify which
-# package ecosystems to update and where the package manifests are located.
-# Please see the documentation for all configuration options:
+# Dependabot configuration
 # https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
 
 version: 2
 updates:
-  - package-ecosystem: "" # See documentation for possible values
-    directory: "/" # Location of package manifests
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "npm"
+    directory: "/site"
     schedule:
       interval: "weekly"