ci: improve workflow triggers to prevent duplicate runs (#163)

- Limit push events to master branch only
- Keep pull_request events for all branches
- This prevents duplicate workflow runs when PRs are created from repository branches

Fixes #162

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <[email protected]>

Author: azu

.github/workflows/test.yml

@@ -1,5 +1,9 @@
 name: test
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 jobs:
   test:
     name: "Test on Node.js ${{ matrix.node-version }} ${{ matrix.os }}"