Fix ci.yml triggering on main when this repo default branch is master

Found immediately after the first real push: the CI badge just added
to README.md showed no status because ci.yml only ever triggered on
push to main, and this repo (all its history predates picking a
default branch name) uses master.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: voidstackloop

.github/workflows/ci.yml

@@ -2,7 +2,7 @@ name: ci
 
 on:
   push:
-    branches: [main]
+    branches: [master]
   pull_request:
 
 jobs: