fix: add CI env var to enable Coveralls upload

The Coveralls formatter in spec_helper.rb requires ENV["CI"] to be set.
When step-level env is specified in GitHub Actions, the default CI
variable from runner level is not inherited, so it must be explicitly added.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: yhk1038

.github/workflows/ci.yml

@@ -74,6 +74,7 @@ jobs:
       - name: Run tests with coverage
         run: bundle exec rspec
         env:
+          CI: true
           COVERAGE: true
           COVERALLS_REPO_TOKEN: ${{ secrets.COVERALLS_REPO_TOKEN }}