ci: add Dependabot for automated dependency updates (#115)

* ci: add Dependabot for automated dependency updates

Configure Dependabot to check bundler dependencies weekly,
keeping Ruby gem dependencies up to date automatically.

* Update .github/dependabot.yml

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

---------

Co-authored-by: greptile-apps[bot] <165735046+greptile-apps[bot]@users.noreply.github.com>

Author: cardmagic

.github/dependabot.yml

@@ -0,0 +1,8 @@
+version: 2
+
+version: 2
+updates:
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "weekly"