ci(terraform): add repository rules as code

[wiktoriavh]

Summary

• Adds terraform/repository/ to manage GitHub repo settings and main branch protection
• Adds terraform-repository workflow to apply on merge to main
• Adds CODEOWNERS for terraform repository changes
• Sets repository visibility to public

Test plan

• terraform fmt -check and terraform validate pass in terraform/repository/
• After merge, verify terraform-repository workflow applies successfully
• Confirm repo visibility is public and branch protection is active on main

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