Add GitHub Actions CI workflow

Same pattern as opencode-ruby's CI. Workflow runs test suite + gem
build + post-install smoke load on Ruby 3.2 / 3.3 / 3.4.

Note: the 'Verify gem loads after install' step depends on
opencode-ruby being resolvable. Once both gems are on rubygems.org
the runtime_dependency resolves automatically; until then, contributors
who want to run the full workflow locally need to either install
opencode-ruby first or rely on the existing 'bundle exec rake test'
step (which uses the Gemfile-resolved dep, not the gem-spec-resolved
one).

.github/workflows/test.yml

@@ -0,0 +1,39 @@
+name: Test
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ["3.2", "3.3", "3.4"]
+    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: Run tests
+        run: bundle exec rake test
+
+      - name: Build gem
+        run: gem build opencode-rails.gemspec
+
+      - name: Verify gem loads after install
+        # opencode-rails depends on opencode-ruby; until both gems
+        # are on rubygems.org, the install step here will only resolve
+        # if opencode-ruby has been pre-installed or is reachable.
+        # When the gems do publish, the runtime_dependency on
+        # opencode-ruby will Just Work via rubygems.
+        run: |
+          gem install --local opencode-rails-*.gem --conservative
+          ruby -ropencode-rails -e 'puts Opencode::RAILS_VERSION'