Configure dependabot for weekly Friday updates

- Schedule gomod and github-actions updates every Friday
- Add github-actions ecosystem to track CI action versions

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: toshi0607

.github/dependabot.yml

@@ -4,3 +4,9 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+      day: "friday"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "friday"