ci: trigger subtree split on config and workflow changes

Include the subtree split config and the workflow file itself in the path
filter so operational changes take effect on the next push instead of
waiting for an unrelated change under project/ or addons/debug/.

Author: gharlan

.github/workflows/subtree-split.yml

@@ -7,6 +7,8 @@ on:
         paths:
             - project/**
             - addons/debug/**
+            - .github/subtree-splits.json
+            - .github/workflows/subtree-split.yml
 
 permissions:
     contents: read