Check every push and pull request

The release workflow gates a tag, which is the last possible moment: a
commit that breaks the suite stayed green on main until somebody decided
to publish, and then failed the release instead of the change that
caused it.

A CI workflow now runs go vet and go test on pushes to main and on pull
requests.

push is scoped to main rather than left open. A branch pushed as part of
a pull request would otherwise be checked twice, and gh-pages carries the
published chart index with no Go code in it, so go vet there would fail
on a missing go.mod.

Runs for the same ref cancel each other, since a rapid series of pushes
only needs the last one checked.

.github/workflows/ci.yml

@@ -0,0 +1,34 @@
+name: CI
+
+# The release workflow gates a tag, which is late: a broken commit sits green
+# until somebody decides to publish. This runs the same checks on the way in.
+#
+# push is scoped to main rather than all branches for two reasons: a branch
+# pushed as part of a pull request would otherwise be checked twice, and
+# gh-pages holds the published Helm chart index with no Go code in it, so
+# `go vet ./...` there would fail on a missing go.mod.
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+# A rapid series of pushes only needs the last one checked.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+
+      - name: Vet
+        run: go vet ./...
+
+      - name: Test
+        run: go test ./...