Simplify CI workflow to single Ubuntu runner

Pure Go project with no CGO — cross-compilation handles all platforms,
no need to test on macOS and Windows runners.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ako

.github/workflows/push-test.yml

@@ -4,16 +4,12 @@ on: [push, pull_request]
 
 jobs:
   build-and-test:
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
-        go-version: ['1.26']
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v4
         with:
-          go-version: ${{ matrix.go-version }}
+          go-version: '1.26'
       - name: Build
         run: make build
       - name: Test