Run CI on all pushes, guard publish to tag-only

Instead of a separate ci.yml (which GitHub Actions won't discover
from a feature branch), consolidate into a single workflow that
triggers on all pushes and PRs. The test jobs always run; the
publish job is guarded with `if: startsWith(github.ref, 'refs/tags/v')`
so it only executes on version tag pushes.

https://claude.ai/code/session_01RBbLduNgvjWctN2Dj12UDY

Author: claude

.github/workflows/ci.yml

@@ -1,9 +1,11 @@
-name: Publish to Hex.pm
+name: CI
 
 on:
   push:
-    tags:
-      - 'v*'
+    branches: ['*']
+    tags: ['v*']
+  pull_request:
+    branches: [main]
 
 jobs:
   test: