chore: add dependabot for automated dependency updates

Configure weekly npm and GitHub Actions dependency updates. Groups
dev and production dependencies separately for easier review.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  - package-ecosystem: npm
+    directory: /
+    schedule:
+      interval: weekly
+    open-pull-requests-limit: 10
+    groups:
+      dev-dependencies:
+        dependency-type: development
+      production-dependencies:
+        dependency-type: production
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly