Add CI: run rspec + standardrb on push and PRs

The repo had no automated checks, so changes to this deploy gem merged
unvalidated. Add a GitHub Actions workflow running the spec suite and standard
across the supported Ruby range (3.2 min per the gemspec, plus 3.3).

Tool: Claude Code (claude-opus-4-8[1m])

Author: JasonRTodd-SRP

.github/workflows/ci.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-22.04
+    strategy:
+      fail-fast: false
+      matrix:
+        # Oldest supported (gemspec required_ruby_version) and current.
+        ruby: ["3.2", "3.3"]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Ruby ${{ matrix.ruby }}
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+
+      - name: RSpec
+        run: bundle exec rspec
+
+      - name: Standard
+        run: bundle exec standardrb