Add RubyGems publish workflow

[hownowstephen]

Summary

• Add GitHub Actions workflow to publish gem to RubyGems on release creation
• Uses OIDC trusted publishing (no API keys/secrets needed)
• Mirrors the publish workflow pattern from customerio-python

Setup required

1. rubygems.org: Add trusted publisher under gem settings — repo customerio/customerio-ruby, workflow publish.yml, environment rubygems
2. GitHub: Create rubygems environment in repo Settings → Environments

Test plan

• Verify workflow YAML is valid
• Configure trusted publisher on rubygems.org
• Create GitHub rubygems environment
• Test with a release publish

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Note

Medium Risk
Introduces an automated release pipeline that can publish the gem to RubyGems using OIDC and write repo contents, so misconfiguration could publish unintended versions or affect release integrity.

Overview
Adds a new GitHub Actions workflow (.github/workflows/publish.yml) that runs on v* tag pushes to build and publish the gem to RubyGems via rubygems/release-gem@v1 using OIDC (id-token: write).

The workflow includes a guard step that fails the run if the pushed tag version does not match Customerio::VERSION before publishing.

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[joeybaer]

@cursor review

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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