ci: run go vet and go test on push/PR

Tests (including the rate-limit suite) previously only ran manually
on the build server; now every push to main and every PR triggers
them via GitHub Actions.

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

.github/workflows/test.yml

@@ -0,0 +1,23 @@
+name: test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.26"
+          cache: true
+
+      - name: Vet
+        run: go vet ./...
+
+      - name: Test
+        run: go test ./...