Add //go:build darwin to secure_enclave.m and secure_enclave.h so Go
ignores them on non-darwin platforms. Without this, the lint stage fails
on Linux with 'Objective-C source files not allowed when not using cgo
or SWIG' because the !darwin stub (macse_stub.go) doesn't use CGO.
- Replaced exec.Command calls to /usr/bin/security with native keybase/go-keychain API
- Added comprehensive test suite for keychain operations
- Fixed binary data storage in tests using hex encoding
- Updated macse tests to skip with explanation about ADE requirements
- All tests passing with CGO_ENABLED=1
