Limit push trigger to main in check-recent workflow

push: branches: '**' caused both the push event (9-worker matrix) and
the pull_request event (1-worker matrix) to fire simultaneously on every
PR commit. Restricting push to [main] means PR pushes only trigger the
pull_request path (1 worker), while direct pushes to main still get the
full 9-worker matrix.

https://claude.ai/code/session_01XHWLohiSTdZvCutwD7ag2b

.github/workflows/check-current-testthat.yaml

@@ -29,10 +29,11 @@
 
 on:
   pull_request:
-    # run in each PR in this repo
+    # run in each PR in this repo (1 worker, see matrix logic below)
     branches: '**'
   push:
-    branches: '**'
+    # only on main; pushing to a PR branch is already covered by pull_request above
+    branches: [main]
   schedule:
     # also run a schedule everyday at 1 AM.
     # this is to check that all dependencies are still available (see R/zzz.R)