chore: group dependabot github-actions updates (#341)

Configures Dependabot to bundle all GitHub Actions ecosystem
updates into a single pull request, reducing PR noise and
simplifying reviews.

.github/dependabot.yaml

diff --git a/.github/dependabot.yaml b/.github/dependabot.yaml
index 1603cdd..e07db06 100644
--- a/.github/dependabot.yaml
+++ b/.github/dependabot.yaml
@@ -7,3 +7,7 @@
     directory: "/"
     schedule:
       interval: "monthly"
+    groups:
+      actions:
+        patterns:
+          - "*"