ci: group Dependabot action updates into one pull request

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CaQrWx6zTzc6GBwwm7PYXQ

Author: chengzeyi

.github/dependabot.yml

@@ -5,3 +5,7 @@ updates:
     directory: /
     schedule:
       interval: weekly
+    # One pull request per run instead of one per action.
+    groups:
+      actions:
+        patterns: ['*']