Reduce CI cost: dedupe triggers, thin macOS matrix (#21)

- Drop push-triggered runs for non-master branches; pull_request
  already covers every PR commit, so push+pull_request was running
  the full 14-job matrix twice per push (28 jobs).
- Only run the full Python 3.8-3.14 matrix on ubuntu-latest. macOS
  runners bill at 10x Linux minutes; test just the oldest and newest
  supported Python there to catch platform-specific issues.
- Net effect: 28 jobs per PR push -> 9 (7 Linux + 2 macOS).

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/run_tests.yml

@@ -1,6 +1,9 @@
 name: Run tests
 
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+    branches: [master]
 
 jobs:
   test:
@@ -8,8 +11,16 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest]
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+        include:
+          # macOS runners cost 10x Linux minutes - only test the oldest
+          # and newest supported Python there to catch platform-specific
+          # issues without running the full matrix twice.
+          - os: macos-latest
+            python-version: "3.8"
+          - os: macos-latest
+            python-version: "3.14"
 
     steps:
     - uses: actions/checkout@v6