Add staticcheck to CI

Caught a real dead-code finding (an unused off() helper) during this
session's stability pass; running it on every push/PR catches this
class of issue automatically instead of relying on someone happening
to run it locally.

.github/workflows/ci.yml

@@ -43,3 +43,9 @@ jobs:
 
       - name: go test
         run: go test ./...
+
+      - name: staticcheck
+        uses: dominikh/staticcheck-action@9716614d4101e79b4340dd97b10e54d68234e431 # v1.4.1
+        with:
+          version: latest
+          install-go: false