Build main weekly

Gemfile.lock is not committed, so a new dependency release can break
the build while main stays unchanged. The json 3.0 release did exactly
that: it broke the Rails 7.x jobs on main (see #766), but no push to
main has happened since, so CI never showed it. Add a weekly scheduled
run so such breakage shows up without waiting for the next push.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: snaka

.github/workflows/ci.yml

@@ -3,6 +3,10 @@ name: split
 on:
   push:
   pull_request:
+  schedule:
+    # Gemfile.lock is not committed, so new dependency releases can break the
+    # build without any change here. Build main weekly to catch that early.
+    - cron: "0 3 * * 1"
 
 jobs:
   test: