### Added
- `/release` skill for automated version releases (#30)
- GitHub Actions CI workflow with multi-version testing matrix (#15)
- Test matrix for Elixir 1.16-1.18 and OTP 26-27
- Codecov integration for coverage reporting
- CI and coverage status badges in README
- `:crypto` application to extra_applications for proper OTP loading
- Keyring behaviour interface with on_encrypt/on_decrypt callbacks (#25)
- Helper functions for data key generation and provider ID validation
- Support for optional plaintext_data_key in materials structs
- DecryptionMaterials.new_for_decrypt/3 and set_plaintext_data_key/2
- EncryptionMaterials.new_for_encrypt/3, set_plaintext_data_key/2, and add_encrypted_data_key/2
- Comprehensive test coverage for keyring behaviour (20 new tests)
- Raw AES keyring implementation with AES-128/192/256 support (#26)
- Provider info serialization for keyring metadata (key name, IV, tag length)
- wrap_key/2 function for encrypting data keys with AES-GCM
- unwrap_key/3 function for decrypting EDKs with provider ID and key name matching
- Comprehensive unit tests (25 tests) and test vector validation (4 vectors)
- Edge case tests for empty/large contexts, unicode, and all key sizes

### Changed
- Minimum Elixir version requirement from 1.18 to 1.16
- Minimum OTP version requirement to 26