chore: add .github/dependabot.yml for automated dependency updates (#130)

Add Dependabot configuration covering npm (weekly, max 5 open PRs) and
github-actions (weekly) ecosystems so security patches and version bumps
are tracked automatically without manual effort.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: minpeter

.github/dependabot.yml

@@ -0,0 +1,12 @@
+version: 2
+updates:
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"