Install pandoc in push_gem workflow for doc generation

The rake release task depends on doc generation which requires pandoc.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

.github/workflows/push_gem.yml

@@ -29,6 +29,9 @@ jobs:
 
       - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
 
+      - name: Install pandoc
+        run: sudo apt-get install -y pandoc
+
       - name: Set up Ruby
         uses: ruby/setup-ruby@d5126b9b3579e429dd52e51e68624dda2e05be25 # v1.267.0
         with: