Migrate CI/CD from CircleCI to GitHub Actions

[khamilowicz]

Summary

• Add .github/workflows/ci.yml: runs build_test, lint, and test on every branch push using reusable workflows from membraneframework/membrane_actions
• Add .github/workflows/release.yml: same CI checks on tags, plus hex_publish after all checks pass
• Remove .circleci/config.yml

Test plan

• Verify CI workflow triggers on branch push and all jobs pass
• Verify release workflow triggers on tag push, CI jobs pass, and hex_publish runs

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